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/+/211277


Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>