spec: Add link to rune literals from string literals when talking about escape sequences.

LGTM=gri
R=golang-codereviews, gobot, gri
CC=golang-codereviews
https://golang.org/cl/140750043

doc/go_spec.html

@@ -479,7 +479,7 @@ Interpreted string literals are character sequences between double
 quotes <code>&quot;&quot;</code>. The text between the quotes,
 which may not contain newlines, forms the
 value of the literal, with backslash escapes interpreted as they
-are in rune literals (except that <code>\'</code> is illegal and
+are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and
 <code>\"</code> is legal), with the same restrictions.
 The three-digit octal (<code>\</code><i>nnn</i>)
 and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual