Fix the test case file path used in a CONTRIBUTING.md example

The path of the test case file used in the example of how to generate
the list of expected lines with errors is wrong. This commit fixes it.

I believe it was changed by mistake in
4a33d46#diff-98e64bc1cd2db9333c6effe87bbe0d6dfe8714aba4c6bde45aa037fe0796e44cR141.

If we want to keep the current path for some reason, we might want to
change the part that says "from the root directory of your WordPressCS
clone".

.github/CONTRIBUTING.md

@@ -138,7 +138,7 @@ Also note the class name convention. The method `getErrorList()` MUST return an
 If you run the following from the root directory of your WordPressCS clone:
 
 ```sh
-$ "vendor/bin/phpcs" --standard=Wordpress -s ./Tests/PHP/POSIXFunctionsUnitTest.inc --sniffs=WordPress.PHP.POSIXFunctions
+$ "vendor/bin/phpcs" --standard=Wordpress -s ./WordPress/Tests/PHP/POSIXFunctionsUnitTest.inc --sniffs=WordPress.PHP.POSIXFunctions
 ...
 --------------------------------------------------------------------------------
 FOUND 7 ERRORS AFFECTING 7 LINES