Fix minor typos in GNUmakefile comment

GNUmakefile

@@ -53,14 +53,11 @@
 # - docker_audit: dynamically builds a docker image and
 #   runs the full suite of audit targets inside it.
 #
-# - docker_rebuild_gramar_tests: dynamically builds a docker image
+# - docker_rebuild_grammar_tests: dynamically builds a docker image
 #   which is then used to reinitialize the "out" files that record
 #   how the grammar parses example inputs, which will then be used
 #   for future grammar audits.
 #
-#   which means future audits will validate the grammar against the
-#   way it presently validates.
-#
 # - audit_grammar: audits the filter language grammar included
 #   in the specification
 #