Downgrade a status message from LOG to DEBUG2.

I never intended this to be anything other than a debugging aid, but forgot
to change the level before committing.

src/backend/access/transam/xlog.c

@@ -9636,7 +9636,7 @@ WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
 		}
 
 		if (currentSource != oldSource)
-			elog(LOG, "switched WAL source from %s to %s after %s",
+			elog(DEBUG2, "switched WAL source from %s to %s after %s",
 				 xlogSourceNames[oldSource], xlogSourceNames[currentSource],
 				 lastSourceFailed ? "failure" : "success");