wa_results_collector: Fix FutureWarning

The warning in question:

"""
data/work/lisa/lisa/wa_results_collector.py:1086: FutureWarning:
Interpreting tuple 'by' as a list of keys, rather than a single
key. Use 'by=[...]' instead of 'by=(...)'. In the future, a tuple will
always mean a single key.
"""

Do what it says.