Docs: Update doc string to not reference deprecated function fillna for ffill (#54728)

* update doc string to not reference deprecated function fillna

* put on one line

Author: sleepypioneer

pandas/core/generic.py

@@ -7368,7 +7368,7 @@ def ffill(
         downcast: dict | None | lib.NoDefault = lib.no_default,
     ) -> Self | None:
         """
-        Synonym for :meth:`DataFrame.fillna` with ``method='ffill'``.
+        Fill NA/NaN values by propagating the last valid observation to next valid.
 
         Returns
         -------