chore: address review comments

Author: olavloite

google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITPgJsonbTest.java

@@ -149,7 +149,7 @@ public void testPgJsonbAsPrimaryKey() {
 
   @Test
   public void testPgJsonbInSecondaryIndex() {
-    // JSONB is not allowed as a primary key.
+    // JSONB is not allowed as a key in a secondary index.
     ExecutionException executionException =
         assertThrows(
             ExecutionException.class,