fixup first/last rendering

Author: MarcoGorelli

spec/API_specification/dataframe_api/dataframe_object.py

@@ -288,8 +288,7 @@ def sorted_indices(
             If a sequence, it must be the same length as `keys`,
             and determines the direction with which to use each
             key to sort by.
-        nulls_position : str
-            Can be either `'first'` or `'last'`.
+        nulls_position : ``{'first', 'last'}``
             Whether null values should be placed at the beginning
             or at the end of the result.
             Note that the position of NaNs is unspecified and may