spec: Correct typo in method expressions example.

Also, remove unnecessary whitespace.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6928045

doc/go_spec.html

@@ -3359,7 +3359,7 @@ these five invocations are equivalent:
 <pre>
 t.Mv(7)
 T.Mv(t, 7)
-(T).Mv(t, t)
+(T).Mv(t, 7)
 f1 := T.Mv; f1(t, 7)
 f2 := (T).Mv; f2(t, 7)
 </pre>