regexp: fix link to RE2 syntax

Fixes #10224

Change-Id: I21037379b4667575e51ab0b6b683138c505c3f68
Reviewed-on: https://go-review.googlesource.com/7960Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/regexp/regexp.go

@@ -7,7 +7,7 @@
 // The syntax of the regular expressions accepted is the same
 // general syntax used by Perl, Python, and other languages.
 // More precisely, it is the syntax accepted by RE2 and described at
-// http://code.google.com/p/re2/wiki/Syntax, except for \C.
+// https://golang.org/s/re2syntax, except for \C.
 // For an overview of the syntax, run
 //   godoc regexp/syntax
 //