docs(guides): contenthash => chunkhash in caching

Correct this error in `output` configuration.

Fixes #1512

Author: skipjack

content/guides/caching.md

@@ -192,7 +192,7 @@ __webpack.config.js__
       })
     ],
     output: {
-      filename: '[name].[contenthash].js',
+      filename: '[name].[chunkhash].js',
       path: path.resolve(__dirname, 'dist')
     }
   };
@@ -319,7 +319,7 @@ __webpack.config.js__
       })
     ],
     output: {
-      filename: '[name].[contenthash].js',
+      filename: '[name].[chunkhash].js',
       path: path.resolve(__dirname, 'dist')
     }
   };