Fix docstring wording

Co-authored-by: David Brochart <[email protected]>

jupyter_ydoc/ybasedoc.py

@@ -117,7 +117,7 @@ def dirty(self, value: bool) -> None:
     @property
     def hash(self) -> Optional[str]:
         """
-        Returns whether the document hash as computed by contents manager.
+        Returns the document hash as computed by contents manager.
 
         :return: The document hash.
         :rtype: Optional[str]