[receiver/postgresqlreceiver] Update golden test fixture for postgresql.backends description

Assisted-by: Claude Sonnet 5

Author: kartikgola

receiver/postgresqlreceiver/testdata/scraper/otel/expected_vector.yaml

@@ -153,7 +153,7 @@ resourceMetrics:
             stringValue: PRAKSURY-M-7HKV:5432
     scopeMetrics:
       - metrics:
-          - description: The number of backends.
+          - description: The number of backend processes associated with each database. Counts backends across all connection states (active, idle, idle-in-transaction) and all backend types, including non-client backends such as autovacuum and parallel workers.
             name: postgresql.backends
             sum:
               aggregationTemporality: 2