Reword NameFormat message for hyphenated_lowercase

It was suggested that this message could be clearer.

Change-Id: I32020f69487c6b9d425ad7150a0ae714ff652752
Reviewed-on: http://gerrit.causes.com/44295
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

lib/scss_lint/linter/name_format.rb

@@ -60,7 +60,7 @@ def check_placeholder(node)
 
     CONVENTIONS = {
       'hyphenated_lowercase' => {
-        explanation: 'in lowercase with hyphens instead of underscores',
+        explanation: 'in all lowercase letters with hyphens instead of underscores',
         validator: ->(name) { name !~ /[_A-Z]/ },
       },
       'BEM' => {