Fix missing comma in get_reference_table doctest example (#275)

The second example block in `get_reference_table`'s docstring was
unrunnable: `collection="parameter-codes"` and `query={...}` were on
separate lines with no comma between them, so the doctest text doesn't
parse as a valid call.

Surfaced from PR #229's verification pass where I executed each public
function's docstring example against the live USGS OGC API.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

dataretrieval/waterdata/api.py

@@ -2050,8 +2050,8 @@ def get_reference_table(
 
         >>> # Get table of selected USGS parameter codes
         >>> ref, md = dataretrieval.waterdata.get_reference_table(
-        ...     collection="parameter-codes"
-        ...     query={'id': '00001,00002'}
+        ...     collection="parameter-codes",
+        ...     query={"id": "00001,00002"},
         ... )
     """
     valid_code_services = get_args(METADATA_COLLECTIONS)