docs(plugins): Add title to cacheGroups (#2227)

addCache test correctly

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[1]: https://cla.js.foundation/webpack/webpack.js.org
[2]: https://webpack.js.org/writers-guide/

Author: sharh

src/content/plugins/split-chunks-plugin.md

@@ -217,6 +217,8 @@ A module can belong to multiple cache groups. The optimization will prefer the c
 
 If the current chunk contains modules already split out from the main bundle, it will be reused instead of a new one being generated. This can impact the resulting file name of the chunk.
 
+#### `splitChunks.cacheGroups.cacheGroup.test`
+
 __webpack.config.js__
 
 ```js
@@ -255,8 +257,8 @@ module.exports = {
           }
         }
       }
-    }
-  }
+		}
+	}
 };
 ```