Fix Sphinx warning about GDScriptLexer

(cherry picked from commit ca680690246a36caf37a1afb4d6fa4305b4084df)

_extensions/gdscript.py

@@ -339,7 +339,7 @@ class GDScriptLexer(RegexLexer):
 
 
 def setup(sphinx):
-    sphinx.add_lexer("gdscript", GDScriptLexer())
+    sphinx.add_lexer("gdscript", GDScriptLexer)
 
     return {
         "parallel_read_safe": True,