os: fix typo in Chmod godoc

Change-Id: I3e5c20d2ffbbe604e6c8b21e2afa50dd6c9f2b7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/176626
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file.go

@@ -492,7 +492,7 @@ func UserHomeDir() (string, error) {
 //
 // On Windows, only the 0200 bit (owner writable) of mode is used; it
 // controls whether the file's read-only attribute is set or cleared.
-// The other bits are currently unused. For compatibilty with Go 1.12
+// The other bits are currently unused. For compatibility with Go 1.12
 // and earlier, use a non-zero mode. Use mode 0400 for a read-only
 // file and 0600 for a readable+writable file.
 //