doc/go_spec: fix a typo

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5989065

doc/go_spec.html

@@ -2396,7 +2396,7 @@ type T2 struct {
 
 func (recv *T2) M2()
 
-var p *T2  // with p != nil and p.T1 != nil
+var p *T2  // with p != nil and p.T0 != nil
 </pre>
 
 <p>