docs: update bigframes.pandas.index docs return types (#1191)

Author: arwas11

third_party/bigframes_vendored/pandas/core/indexes/base.py

@@ -987,7 +987,7 @@ def drop(self, labels) -> Index:
             labels (array-like or scalar):
 
         Returns:
-            Index: Will be same type as self.
+            bigframes.pandas.Index: Will be same type as self.
         """
         raise NotImplementedError(constants.ABSTRACT_METHOD_ERROR_MESSAGE)