fix: find previous exception due to wrong iteration direction

Studio/CelesteStudio/Dialog/FindDialog.cs

@@ -78,7 +78,7 @@ private void SelectPrev() {
             return;
 
         // Check against end of selection
-        for (int i = matches.Count - 1; i >= 0; i++) {
+        for (int i = matches.Count - 1; i >= 0; i--) {
             var match = matches[i];
             var end = new CaretPosition(match.Row, match.Col + textBox.Text.Length);