Fix README example (flag_specs -> flags)

[justinlubin]

I just tried running the README example on Gleam 1.18.1 and it gave me a type error; I believe the fix is to change flag_specs to flags on Line 56 of the README (that seems to fix it in my project).

Here is my manifest.toml:

packages = [
  { name = "argv", version = "1.1.0", build_tools = ["gleam"], requirements = [], otp_app = "argv", source = "hex", outer_checksum = "3277D100448BDB4A29B6D58C0F36F631CBC349E8BDD09766C6309DF202831140" },
  { name = "gleam_community_ansi", version = "1.5.0", build_tools = ["gleam"], requirements = ["gleam_community_colour", "gleam_regexp", "gleam_stdlib"], otp_app = "gleam_community_ansi", source = "hex", outer_checksum = "B5AA433AF84313E23FDF90CCFF752B9380FE9FFCE02B2949D49B7AACCC77B16D" },
  { name = "gleam_community_colour", version = "2.0.4", build_tools = ["gleam"], requirements = ["gleam_json", "gleam_stdlib"], otp_app = "gleam_community_colour", source = "hex", outer_checksum = "6DB4665555D7D2B27F0EA32EF47E8BEBC4303821765F9C73D483F38EE24894F0" },
  { name = "gleam_json", version = "3.1.0", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "gleam_json", source = "hex", outer_checksum = "44FDAA8847BE8FC48CA7A1C089706BD54BADCC4C45B237A992EDDF9F2CDB2836" },
  { name = "gleam_regexp", version = "1.1.1", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "gleam_regexp", source = "hex", outer_checksum = "9C215C6CA84A5B35BB934A9B61A9A306EC743153BE2B0425A0D032E477B062A9" },
  { name = "gleam_stdlib", version = "1.0.5", build_tools = ["gleam"], requirements = [], otp_app = "gleam_stdlib", source = "hex", outer_checksum = "CEE5B6C076A85B45F60C585F4316C63EC8B7127C119D5738C3958A9C4D50404E" },
  { name = "gleeunit", version = "1.11.0", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "gleeunit", source = "hex", outer_checksum = "EC31ABA74256AEA531EDF8169931D775BBB384FED0A8A1BDC4DD9354E3E21826" },
  { name = "glint", version = "1.3.0", build_tools = ["gleam"], requirements = ["gleam_community_ansi", "gleam_community_colour", "gleam_stdlib", "snag"], otp_app = "glint", source = "hex", outer_checksum = "26CCA9BC3ACB56CD9D754ACF598F49B4C281C4FDD81AE61FA21C8E92E9C8218E" },
  { name = "hoist", version = "1.0.0", build_tools = ["gleam"], requirements = ["gleam_regexp", "gleam_stdlib"], otp_app = "hoist", source = "hex", outer_checksum = "FDB18604BA29BD58809DCA2D05674DD70123645491DBB4E8B8ACC4A06DB642AC" },
  { name = "snag", version = "1.2.0", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "snag", source = "hex", outer_checksum = "274F41D6C3ECF99F7686FDCE54183333E41D2C1CA5A3A673F9A8B2C7A4401077" },
]

[requirements]
argv = { version = ">= 1.1.0 and < 2.0.0" }
gleam_stdlib = { version = ">= 1.0.0 and < 2.0.0" }
gleeunit = { version = ">= 1.0.0 and < 2.0.0" }
glint = { version = ">= 1.3.0 and < 2.0.0" }
hoist = { version = ">= 1.0.0 and < 2.0.0" }

Thanks for the lovely package!