fix message and category

Author: y

lib/rules/prop-name-casing.js

@@ -40,7 +40,7 @@ function create (context) {
       if (convertedName !== propName) {
         context.report({
           node: item,
-          message: 'Prop "{{name}}" is not {{caseType}}.',
+          message: 'Prop "{{name}}" is not in {{caseType}}.',
           data: {
             name: propName,
             caseType: caseType
@@ -59,7 +59,7 @@ module.exports = {
   meta: {
     docs: {
       description: 'enforce specific casing for the Prop name in Vue components',
-      category: 'strongly-recommended'
+      category: 'upcoming'
     },
     fixable: null,  // or "code" or "whitespace"
     schema: [