spec: clarify when a program exits

Fixes #8023.

LGTM=rsc
R=r, iant, ken, rsc
CC=golang-codereviews
https://golang.org/cl/98340043

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 14, 2014",
+	"Subtitle": "Version of May 19, 2014",
 	"Path": "/ref/spec"
 }-->
 
@@ -5979,7 +5979,7 @@ func main() { … }
 <p>
 Program execution begins by initializing the main package and then
 invoking the function <code>main</code>.
-When the function <code>main</code> returns, the program exits.
+When that function invocation returns, the program exits.
 It does not wait for other (non-<code>main</code>) goroutines to complete.
 </p>