Fix typos in source code using codespell

From https://github.com/lucasdemarchi/codespell

scene/gui/box_container.cpp

@@ -98,7 +98,7 @@ void BoxContainer::_resort() {
 		elements exist */
 
 	bool has_stretched = false;
-	while (stretch_ratio_total > 0) { // first of all, dont even be here if no stretchable objects exist
+	while (stretch_ratio_total > 0) { // first of all, don't even be here if no stretchable objects exist
 
 		has_stretched = true;
 		bool refit_successful = true; //assume refit-test will go well