[php]: Fix documentation model header (#23306)

Co-authored-by: coffeemakr <coffeemakr@users.noreply.github.com>

Author: coffeemakr

modules/openapi-generator/src/main/resources/php-nextgen/model_doc.mustache

@@ -1,4 +1,4 @@
-# {{#models}}{{#model}}# {{classname}}
+{{#models}}{{#model}}# {{classname}}
 
 ## Properties
 

modules/openapi-generator/src/main/resources/php/model_doc.mustache

@@ -1,4 +1,4 @@
-# {{#models}}{{#model}}# {{classname}}
+{{#models}}{{#model}}# {{classname}}
 
 ## Properties