cmd/gofmt: unindent the second line of a BUG note

Currently, this second line is treated a pre-formatted text as it's
indented relatively to the BUG() line.

The current state can be seen at:

  https://golang.org/cmd/gofmt/#pkg-note-BUG

Unindenting makes the rest of the sentence part of the same paragraph.

Change-Id: I6dee55c9c321b1a03b41c7124c6a1ea15772c878
Reviewed-on: https://go-review.googlesource.com/38353Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>