Fix typo in comment.

bin/brew

@@ -412,7 +412,7 @@ rescue BuildError => e
   onoe e
   puts PLEASE_REPORT_BUG
   # this feature can be slow (depends on network conditions and if github is up)
-  # so ideally we'd show feedback, eg. "checking for existin issues..." and
+  # so ideally we'd show feedback, eg. "checking for existing issues..." and
   # then replace that string with the following when the github api returns
   issues = issues_for_formula(formula_name)
   puts "These existing issues may help you:", *issues unless issues.empty?