fix typo found by kevin ballard

R=gri
CC=golang-dev
https://golang.org/cl/186113

doc/go_spec.html

@@ -2458,7 +2458,7 @@ More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> a
 that the dynamic type of <code>x</code> is identical to the type <code>T</code>
 (§<a href="#Type_identity_and_compatibility">Type identity and compatibility</a>).
 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
-of <code>T</code> implements the interface <code>T</code> (§<a href="#Interface_types">Interface types</a>).
+of <code>x</code> implements the interface <code>T</code> (§<a href="#Interface_types">Interface types</a>).
 </p>
 <p>
 If the type assertion holds, the value of the expression is the value