Update README for simplified rule ref

Author: sirbrillig

README.md

@@ -33,7 +33,7 @@ composer require --dev sirbrillig/phpcs-variable-analysis
 You can then include the sniffs by adding a line like the following to [your phpcs.xml file](https://github.com/squizlabs/PHP_CodeSniffer/wiki/Advanced-Usage#using-a-default-configuration-file).
 
 ```
-<rule ref="phpcs-variable-analysis.CodeAnalysis.VariableAnalysis"/>
+<rule ref="VariableAnalysis"/>
 ```
 
 It should just work after that!