fix typing, another try

Author: kmuehlbauer

xarray/core/options.py

@@ -267,8 +267,8 @@ class set_options:
     warn_for_unclosed_files : bool, default: False
         Whether or not to issue a warning when unclosed files are
         deallocated. This is mostly useful for debugging.
-    time_resolution : {"s", "ms", "us", "ns"}, default: "ns"
-        Time resolution used for CF encoding/decoding.
+    time_resolution : str, default: "ns"
+        Time resolution used for CF encoding/decoding, one of {"s", "ms", "us", "ns"}.
 
     Examples
     --------