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 <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
Trust: Jay Conrod <[email protected]>
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>

src/cmd/go/internal/work/build.go

@@ -128,8 +128,8 @@ and test commands:
 		a build will run as if the disk file path exists with the contents
 		given by the backing file paths, or as if the disk file path does not
 		exist if its backing file path is empty. Support for the -overlay flag
-		has some limitations:importantly, cgo files included from outside the
-		include path must be  in the same directory as the Go package they are
+		has some limitations: importantly, cgo files included from outside the
+		include path must be in the same directory as the Go package they are
 		included from, and overlays will not appear when binaries and tests are
 		run through go run and go test respectively.
 	-pkgdir dir