spec: use the term "lexical token" (rather then "lexical symbol")

Fix for minor inconsistency.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5709058

doc/go_spec.html

@@ -67,7 +67,7 @@ operators, in increasing precedence:
 
 <p>
 Lower-case production names are used to identify lexical tokens.
-Non-terminals are in CamelCase. Lexical symbols are enclosed in
+Non-terminals are in CamelCase. Lexical tokens are enclosed in
 double quotes <code>""</code> or back quotes <code>``</code>.
 </p>