spec: add () to method calls in examples

Since p.M is rejected (must call methods), use p.M(),
to keep the examples compiling.

Fixes #4441.

R=gri
CC=golang-dev
https://golang.org/cl/6854096

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of November 21, 2012",
+	"Subtitle": "Version of November 26, 2012",
 	"Path": "/ref/spec"
 }-->
 
@@ -2463,9 +2463,9 @@ p.z   // (*p).z
 p.y   // ((*p).T1).y
 p.x   // (*(*p).T0).x
 
-p.M2  // (*p).M2
-p.M1  // ((*p).T1).M1
-p.M0  // ((*p).T0).M0
+p.M2()  // (*p).M2()
+p.M1()  // ((*p).T1).M1()
+p.M0()  // ((*p).T0).M0()
 </pre>