fix generate matrix for olap for saturdays (#10622)

## Problem

when introducing pg17 for job step `Generate matrix for OLAP benchmarks`
I introduced a syntax error that only hits on Saturdays.

## Summary of changes

Remove trailing comma

## successful test run

https://github.com/neondatabase/neon/actions/runs/13086363907

.github/workflows/benchmarking.yml

@@ -341,7 +341,7 @@ jobs:
           ],
           "pg_version" : [
             16,17
-          ],
+          ]
         }'
 
         if [ "$(date +%A)" = "Saturday" ] || [ ${RUN_AWS_RDS_AND_AURORA} = "true" ]; then