Fix formatting of the "pex.platforms is deprecated" message (#20514)

Fixes a missing f-string formatter for the `pex_binary`'s `platforms`
field

Before:
> 16:02:15.59 [WARN] DEPRECATED: the platforms field is scheduled to be
removed in version 2.22.0.dev0.
Using the `platforms` field in the target
src/python/something_something:pex-executable. The platforms field is a
hack. The abbreviated information it provides is sometimes insufficient,
leading to hard-to-debug build issues. Use complete_platforms instead.
See {doc_url('pex')} for details.

After:
> 16:03:43.09 [WARN] DEPRECATED: the platforms field is scheduled to be
removed in version 2.22.0.dev0.
Using the `platforms` field in the target
src/python/something_something:pex-executable. The platforms field is a
hack. The abbreviated information it provides is sometimes insufficient,
leading to hard-to-debug build issues. Use complete_platforms instead.
See https://www.pantsbuild.org/v2.20/docs/pex for details.

Author: lowvoltage

src/python/pants/backend/python/target_types.py

@@ -455,7 +455,7 @@ class PexPlatformsField(StringSequenceField):
     alias = "platforms"
     removal_version = "2.22.0.dev0"
     removal_hint = softwrap(
-        """\
+        f"""\
     The platforms field is a hack. The abbreviated information it provides is sometimes insufficient,
     leading to hard-to-debug build issues. Use complete_platforms instead.
     See {doc_url('pex')} for details.