Drop double \ from parameter types (#445)

Currently the generation of files is failing due to ?\\DateTimeZone in a nullable parameter on at least one function. This changeset fixes that a bit crudely but effectively.

generator/src/WritePhpFunction.php

@@ -170,7 +170,7 @@ private function displayParamsWithType(array $params): string
             $paramsAsString[] = $paramAsString;
         }
 
-        return implode(', ', $paramsAsString);
+        return str_replace('\\\\', '\\', implode(', ', $paramsAsString));
     }
 
     private function printFunctionCall(Method $function): string