Minor documentation change on allowing checkers besides sparse

This patch cleans up a couple of mentions of sparse in the inline
toplevel Makefile documentation such that it's clear that other checkers
besides sparse can override CHECK and CHECKFLAGS.

Signed-off-by: Dustin Kirkland <dustin.kirkland@us.ibm.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>