Fix comment; checkpointer, not bgwriter, performs checkpoints since 9.2.

Amit Langote

src/backend/access/transam/xlog.c

@@ -128,7 +128,7 @@ const struct config_enum_entry sync_method_options[] = {
 
 /*
  * Statistics for current checkpoint are collected in this global struct.
- * Because only the background writer or a stand-alone backend can perform
+ * Because only the checkpointer or a stand-alone backend can perform
  * checkpoints, this will be unused in normal backends.
  */
 CheckpointStatsData CheckpointStats;