go spec: fix error in production syntax

Fix analoguous error in ebnf.go which already
correctly accepted an empty production.

Fixes #1821.

R=r
CC=golang-dev
https://golang.org/cl/4526056

doc/go_spec.html

 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of May 12, 2011 -->
+<!-- subtitle Version of May 13, 2011 -->
 
 <!--
 TODO
@@ -46,7 +46,7 @@ The syntax is specified using Extended Backus-Naur Form (EBNF):
 </p>
 
 <pre class="grammar">
-Production  = production_name "=" Expression "." .
+Production  = production_name "=" [ Expression ] "." .
 Expression  = Alternative { "|" Alternative } .
 Alternative = Term { Term } .
 Term        = production_name | token [ "..." token ] | Group | Option | Repetition .

src/pkg/ebnf/ebnf.go

@@ -5,7 +5,7 @@
 // Package ebnf is a library for EBNF grammars. The input is text ([]byte)
 // satisfying the following grammar (represented itself in EBNF):
 //
-//	Production  = name "=" Expression "." .
+//	Production  = name "=" [ Expression ] "." .
 //	Expression  = Alternative { "|" Alternative } .
 //	Alternative = Term { Term } .
 //	Term        = name | token [ "..." token ] | Group | Option | Repetition .