encoding/base64: fix copy-paste-o bug in RawURLEncoding docs

Fixes #12244

Change-Id: Iee4e45d9bca0718c71fcc574bc51b2084c3dcb2a
Reviewed-on: https://go-review.googlesource.com/13783Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/encoding/base64/base64.go

@@ -75,7 +75,7 @@ var URLEncoding = NewEncoding(encodeURL)
 // This is the same as StdEncoding but omits padding characters.
 var RawStdEncoding = StdEncoding.WithPadding(NoPadding)
 
-// URLEncoding is the unpadded alternate base64 encoding defined in RFC 4648.
+// RawURLEncoding is the unpadded alternate base64 encoding defined in RFC 4648.
 // It is typically used in URLs and file names.
 // This is the same as URLEncoding but omits padding characters.
 var RawURLEncoding = URLEncoding.WithPadding(NoPadding)