flag: removed the colon after "Usage" in the documentation
flag: removed the colon after "Usage" in the documentation

Removing the colon will make the "Usage" and "Command line flag syntax" in the docs, a header when interpreted by godoc.

Fixes #25749

Change-Id: Ifc5572e171db1aaef9775b1d6c86091a8f2528fd
GitHub-Last-Rev: 1b579734308cbcb96f17d945b9c7af70e259cb5b
GitHub-Pull-Request: golang/go#25750
Reviewed-on: https://go-review.googlesource.com/116555
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

akutz/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,akutz/go,cloudflare/go,josharian/go,nawawi/go,akutz/go,akutz/go,golang/go,josharian/go,golang/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,josharian/go,nawawi/go,nawawi/go,nawawi/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,akutz/go,josharian/go,josharian/go,josharian/go,CAFxX/go,golang/go,golang/go,josharian/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go