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/116555Reviewed-by: Rob Pike <r@golang.org>