Fix comment of StrategySyncStart()

The top comment of StrategySyncStart() mentions BufferSync(), but this
function calls BgBufferSync(), not BufferSync().

Oversight in 9cd00c457e.

Author: Ashutosh Bapat
Discussion: https://postgr.es/m/CAExHW5tgkjag8i-s=RFrCn5KAWDrC4zEPPkfUKczfccPOxBRQQ@mail.gmail.com
Backpatch-through: 13

src/backend/storage/buffer/freelist.c

@@ -380,10 +380,10 @@ StrategyFreeBuffer(BufferDesc *buf)
 }
 
 /*
- * StrategySyncStart -- tell BufferSync where to start syncing
+ * StrategySyncStart -- tell BgBufferSync where to start syncing
  *
  * The result is the buffer index of the best buffer to sync first.
- * BufferSync() will proceed circularly around the buffer array from there.
+ * BgBufferSync() will proceed circularly around the buffer array from there.
  *
  * In addition, we return the completed-pass count (which is effectively
  * the higher-order bits of nextVictimBuffer) and the count of recent buffer