[readme] clear up Fragment docs

Fixes #2818

Author: ljharb

README.md

@@ -39,7 +39,7 @@ You should also specify settings that will be shared across all the plugin rules
       "createClass": "createReactClass", // Regex for Component Factory to use,
                                          // default to "createReactClass"
       "pragma": "React",  // Pragma to use, default to "React"
-      "fragment": "React.Fragment",  // Fragment to use, default to "React.Fragment"
+      "fragment": "Fragment",  // Fragment to use (may be a property of <pragma>), default to "Fragment"
       "version": "detect", // React version. "detect" automatically picks the version you have installed.
                            // You can also use `16.0`, `16.3`, etc, if you want to override the detected value.
                            // default to latest and warns if missing