Make docstring a bit more concise

Author: takluyver

src/pyproject_hooks/_impl.py

@@ -35,7 +35,7 @@ def __init__(self, backend_name, backend_path, message):
 
 
 class HookMissing(Exception):
-    """Will be raised on missing hooks (if a default/fallback can't be used)."""
+    """Will be raised on missing hooks (if a fallback can't be used)."""
     def __init__(self, hook_name):
         super().__init__(hook_name)
         self.hook_name = hook_name