cmd/go/internal/cache: fix wrong/old function name in comment

Change-Id: Ia0caf2fb06097ac184f78779334460900e8c0149
Reviewed-on: https://go-review.googlesource.com/c/148580Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/cache/hash.go

@@ -123,7 +123,7 @@ var hashFileCache struct {
 	m map[string][HashSize]byte
 }
 
-// HashFile returns the hash of the named file.
+// FileHash returns the hash of the named file.
 // It caches repeated lookups for a given file,
 // and the cache entry for a file can be initialized
 // using SetFileHash.