Change the default shortcut for git blame on Linux (#13637)

Zed already has a shortcut assigned to ctrl-alt-g and it's mapped to
`search::SelectNextMatch`. Having another multi shortcut with the same
prefix makes `ctrl-alt-g` to have a very noticeable delay when pressed.

This commit changes the default shortcut for git blame to `alt-g b`

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -100,7 +100,7 @@
       "ctrl-k ctrl-r": "editor::RevertSelectedHunks",
       "ctrl-'": "editor::ToggleHunkDiff",
       "ctrl-\"": "editor::ExpandAllHunkDiffs",
-      "ctrl-alt-g b": "editor::ToggleGitBlame"
+      "alt-g b": "editor::ToggleGitBlame"
     }
   },
   {