Fix: Incorrect f-string in previews.py

Contributed by @deadpin

Differential Revision: https://developer.blender.org/D7458

release/scripts/modules/bpy/utils/previews.py

@@ -94,7 +94,7 @@ class ImagePreviewCollection(dict):
 
     def load(self, name, path, path_type, force_reload=False):
         if name in self:
-            raise KeyError("key {name!r} already exists")
+            raise KeyError(f"key {name!r} already exists")
         p = self[name] = _utils_previews.load(
             self._gen_key(name), path, path_type, force_reload)
         return p