cmd/go: fix spacing in help text of -overlay flag

There was a space missing in the first line shown below, and an extra
space in the second line shown. Thanks Peter Bourgon for noting this.

BEFORE:
$ go help build | grep -A1 'has some limitations'
has some limitations:importantly, cgo files included from outside the
include path must be  in the same directory as the Go package they are

AFTER:
$ go help build | grep -A1 'has some limitations'
has some limitations: importantly, cgo files included from outside the
include path must be in the same directory as the Go package they are

Note that I edited alldocs.go by hand here, as the mkalldocs.sh script
produces a lot more changes, for example adding the -insecure flag
documentation in. Not sure what's wrong there.

Change-Id: I303f6d6b42b0e24cec0748a949dc23beec64b917
Reviewed-on: https://go-review.googlesource.com/c/go/+/319949


Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>