fix README

Author: korelstar

README.md

@@ -4,7 +4,7 @@ This problem matcher lets you show errors from `xmllint` as annotation in GitHub
 
 ## Usage
 
-Add the step to your workflow, before PHPUnit is called.
+Add the step to your workflow, before `xmllint` is called.
 
 ```yaml
     - uses: korelstar/xmllint-problem-matcher@v1