fixed violation of naming convention (file=class=function names must …

…be identical) - this also removes one classname collision with 'HFile_plain'

see \ilPCSourceCode::highlightText regarding the naming convention

Services/COPage/syntax_highlight/php/HFile/HFile_mixedphp.php

@@ -3,9 +3,9 @@
 if (!isset ($BEAUT_PATH)) return;
 require_once("$BEAUT_PATH/Beautifier/HFile.php");
 
-class HFile_plain extends HFile{
+class HFile_mixedphp extends HFile{
 
- function HFile_plain(){
+ function HFile_mixedphp(){
 
     $this->HFile();