cmd/go: fix a typo.

Change-Id: Ic453da17817f66e1073f6ba740b830b8daf2fc38
Reviewed-on: https://go-review.googlesource.com/8829Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/build.go

@@ -76,7 +76,7 @@ and test commands:
 		print the commands.
 
 	-buildmode mode
-		build mode to use. See 'go help buildmodes' for more.
+		build mode to use. See 'go help buildmode' for more.
 	-linkshared
 		link against shared libraries previously created with
 		-buildmode=shared

src/cmd/go/doc.go

@@ -96,7 +96,7 @@ and test commands:
 		print the commands.
 
 	-buildmode mode
-		build mode to use. See 'go help buildmodes' for more.
+		build mode to use. See 'go help buildmode' for more.
 	-linkshared
 		link against shared libraries previously created with
 		-buildmode=shared