Use double quotes to avoid ambiguity with tags

Author: jparise

doc/graphql.txt

@@ -22,7 +22,7 @@ installed, GraphQL syntax support in ES6 tagged template literals is enabled.
                                                    *g:graphql_javascript_tags*
 |g:graphql_javascript_tags|                                    list of strings
 
-  Default: `['gql', 'graphql', 'Relay.QL']`
+  Default: `["gql", "graphql", "Relay.QL"]`
 
   This variable lists the JavaScript/ES6 template literal tag names that will
   be recognized as containing GraphQL template strings.