html: fix typo in UnescapeString string docs

Fixes #15221

Change-Id: I9e927a2f604213338b4572f1a32d0247c58bdc60
Reviewed-on: https://go-review.googlesource.com/21798
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: bradfitz

src/html/escape.go

@@ -181,7 +181,7 @@ func EscapeString(s string) string {
 
 // UnescapeString unescapes entities like "&lt;" to become "<". It unescapes a
 // larger range of entities than EscapeString escapes. For example, "&aacute;"
-// unescapes to "á", as does "&#225;" and "&xE1;".
+// unescapes to "á", as does "&#225;" and "&#xE1;".
 // UnescapeString(EscapeString(s)) == s always holds, but the converse isn't
 // always true.
 func UnescapeString(s string) string {