Typo in pg_stat_activity definition

I noticed an inconsistency between uppercase and lowercase letters in only
one place of pg_stat_activity definition, so I'll report it. I think this
is just a typo and there is no functionality issue.
When I compared to the content before and after, I think the capital letter
"S" than "s" is suitable for alias, as shown below.

```diff
diff --git a/src/backend/catalog/system_views.sql
b/src/backend/catalog/system_views.sql
index 08f780a2e63..e5dbbe61b81 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -895,7 +895,7 @@ CREATE VIEW pg_stat_activity AS
             S.wait_event,
             S.state,
             S.backend_xid,
-            s.backend_xmin,
+            S.backend_xmin,
             S.query_id,
             S.query,
             S.backend_type
```

Regards