cmd/compile: fix wording in README

"Syntax analysis" sounds more familiar and fits the
item before, which says "lexical analysis".
If there was specific intention to the original wording,
I, as a reader, would like to see it instead of this
confusing wording.

Change-Id: Id32dbf75300a86b21cb9f35e54526184fe5df6cb
Reviewed-on: https://go-review.googlesource.com/117696


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