keymap: Fix shift- modifier symbol (#25238)

Closes #25230

```
The shift- modifier can only be used in combination with a letter to indicate the uppercase version. For example shift-g matches typing G. Although on many keyboards shift is used to type punctuation characters like (, the keypress is not considered to be modified and so shift-( does not match.
```

[Document](https://zed.dev/docs/key-bindings#keybinding-syntax)

Release Notes:

- Fixed Keymap use `shift-` modifier symbol

crates/zed/src/zed.rs

@@ -3964,7 +3964,7 @@ mod tests {
         assert_key_bindings_for(
             workspace.into(),
             cx,
-            vec![("backspace", &B), ("[", &ActivatePrevItem)],
+            vec![("backspace", &B), ("{", &ActivatePrevItem)],
             line!(),
         );
     }