docs: Fix the update() docstring

`update()` raises `ValueError` for any key that is not an updatable field,
not only for `type` and `dry_run`. Also fixes a "preceeded" typo.

Signed-off-by: Mathias L. Baumann <mathias.baumann@frequenz.com>

Author: Marenz

src/frequenz/client/dispatch/_client.py

@@ -382,7 +382,7 @@ async def update(  # pylint: disable=too-many-branches
         The `new_fields` argument is a dictionary of fields to update. The keys are
         the field names, and the values are the new values for the fields.
 
-        For recurrence fields, the keys are preceeded by "recurrence.".
+        For recurrence fields, the keys are preceded by "recurrence.".
 
         Note that updating `type` and `dry_run` is not supported.
 
@@ -395,7 +395,8 @@ async def update(  # pylint: disable=too-many-branches
             Dispatch: The updated dispatch.
 
         Raises:
-            ValueError: If updating `type` or `dry_run`.
+            ValueError: If `new_fields` contains a key that is not an updatable
+                field, which includes `type` and `dry_run`.
         """
         msg = UpdateMicrogridDispatchRequest(
             dispatch_id=int(dispatch_id), microgrid_id=int(microgrid_id)