spec: add dropped comma

The proposed text in the last CL had a comma that was missing from the submitted spec.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/150720043

doc/go_spec.html

@@ -5252,7 +5252,7 @@ the surrounding function returns, in the reverse order
 they were deferred.
 If a deferred function value evaluates
 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
-when the function is invoked not when the "defer" statement is executed.
+when the function is invoked, not when the "defer" statement is executed.
 </p>
 
 <p>