add codeblock indentation back to --dump-extension-api-with-docs

xml codeblock indentation was changed from spaces to tabs but the tabs
were being replaced with empty strings when exported with
--dump-extension-api-with-docs, this is small change so that tab
characters are no longer replaced.

(cherry picked from commit e9131550b1)

core/extension/extension_api_dump.cpp

@@ -96,7 +96,7 @@ static String fix_doc_description(const String &p_bbcode) {
 	// Based on what EditorHelp does.
 
 	return p_bbcode.dedent()
-			.remove_chars("\t\r")
+			.remove_chars("\r")
 			.strip_edges();
 }