Correct vue.config.js example (#1096)

A bracket is missing from the vue.config.js configuration example, this change would add in the missing bracket so the code example is valid.

Author: lindsaykwardell

src/guide/migration/migration-build.md

@@ -91,6 +91,7 @@ The following workflow walks through the steps of migrating an actual Vue 2 app
              }
            }
          })
+     }
    }
    ```