fix doc isna (#37192)

Author: erfannariman

pandas/core/generic.py

@@ -7309,7 +7309,7 @@ def isna(self: FrameOrSeries) -> FrameOrSeries:
         -------
         {klass}
             Mask of bool values for each element in {klass} that
-            indicates whether an element is not an NA value.
+            indicates whether an element is an NA value.
 
         See Also
         --------