Fix mergeable docs to be consistent with checks (#7021)

.github/mergeable.yml

@@ -9,10 +9,10 @@ mergeable:
           - and:
             - must_include:
                 regex: 'release notes: yes'
-                message: 'Please include release note: yes'
+                message: 'Please include release notes: yes'
             - must_include:
                 regex: '^(c#|c\+\+|cleanup|conformance tests|integration|java|javascript|go|objective-c|php|python|ruby)'
                 message: 'Please include at least a language label (e.g., c++, java, python). Or apply one of the following labels: cleanup, conformance tests, integration.'
           - must_include:
               regex: 'release notes: no'
-              message: 'Please include release note: no'
+              message: 'Please include release notes: no'