The document `doc/go_spec.html` uses "preceeding" instead of the word
"preceding" in one place.

Fixed another occurrence in `src/go/types/typexpr.go`.

Change-Id: Ic67f62026b5c9d002c5c5632299f14ecac8b02ae
Reviewed-on: https://go-review.googlesource.com/12354Reviewed-by: Ian Lance Taylor <iant@golang.org>