PSR12 ruleset: minor documentation tweak

Noticed this reference was missing while reviewing 233.

Author: jrfnl

src/Standards/PSR12/ruleset.xml

@@ -96,6 +96,7 @@
     <!-- When a file contains a mix of HTML and PHP, any of the above sections may still be used. If so, they MUST be present at the top of the file, even if the remainder of the code consists of a closing PHP tag and then a mixture of HTML and PHP. -->
 
     <!-- When the opening php tag is on the first line of the file, it MUST be on its own line with no other statements unless it is a file containing markup outside of PHP opening and closing tags. -->
+    <!-- checked by PSR12.Files.OpenTag -->
 
     <!-- Import statements MUST never begin with a leading backslash as they must always be fully qualified. -->
     <!-- checked by PSR12.Files.ImportStatement -->