spec: add missing comma.

A real humdinger.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4538089

doc/go_spec.html

@@ -1696,7 +1696,7 @@ of an interface type or of elements of a composite type remains unchanged:
 </p>
 
 <pre>
-// A Mutex is a data type with two methods Lock and Unlock.
+// A Mutex is a data type with two methods, Lock and Unlock.
 type Mutex struct         { /* Mutex fields */ }
 func (m *Mutex) Lock()    { /* Lock implementation */ }
 func (m *Mutex) Unlock()  { /* Unlock implementation */ }