spec: reword claim about the grammar being regular

Since the word "regular" has a precise meaning in the context of
formal languages, the Introduction sentence claiming that Go's grammar
is "compact and regular" may mislead readers.

Reword it using Rob's suggestion.

Fixes #36037

Change-Id: I00c1a5714bdab8878d9a77b36d67dae67d63da0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/211277Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Sep 4, 2019",
+	"Subtitle": "Version of Dec 13, 2019",
 	"Path": "/ref/spec"
 }-->
 
@@ -20,8 +20,8 @@ dependencies.
 </p>
 
 <p>
-The grammar is compact and regular, allowing for easy analysis by
-automatic tools such as integrated development environments.
+The grammar is compact and simple to parse, allowing for easy analysis
+by automatic tools such as integrated development environments.
 </p>
 
 <h2 id="Notation">Notation</h2>