docs: Fix build command in deployment documentation

[mostlyinvisible]

The Self Contained Executable instructions in the Deployment documentation contains an error in the sample code.

$ deno build
$ deno compile --include static --include _fresh --include deno.json -A main.ts

should be:

$ deno task build
$ deno compile --include static --include _fresh --include deno.json -A main.ts

This PR fixes that.

[marvinhagemeister]

I think both work since deno also supports calling tasks directly that way. That said I kinda like the consistency of deno task <whatever> more.