chore(spanner): lint fix

Author: harshachinta

.github/workflows/ci.yaml

@@ -95,7 +95,7 @@ jobs:
           JOB_TYPE: test
           GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS: true
           GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_PARTITIONED_OPS: true
-        GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW: true
+          GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW: true
   windows:
     runs-on: windows-latest
     steps: