Fix typos 'a' and 'an'

modules/visual_script/visual_script_expression.cpp

@@ -1179,7 +1179,7 @@ VisualScriptExpression::ENode *VisualScriptExpression::_parse_expression() {
 			if (expression[next_op + 1].is_op) {
 				// this is not invalid and can really appear
 				// but it becomes invalid anyway because no binary op
-				// can be followed by an unary op in a valid combination,
+				// can be followed by a unary op in a valid combination,
 				// due to how precedence works, unaries will always disappear first
 
 				_set_error("Unexpected two consecutive operators.");