Repair out-of-date information in src/backend/storage/buffer/README.

In commit d526575, we changed things so
that buffer usage counts are incremented when the buffer is pinned, rather
than when it is unpinned, but the README file didn't get the memo.

Report by Amit Kapila.

Author: robertmhaas

src/backend/storage/buffer/README

@@ -169,8 +169,8 @@ need to take system-wide locks during common operations.  It works like
 this:
 
 Each buffer header contains a usage counter, which is incremented (up to a
-small limit value) whenever the buffer is unpinned.  (This requires only the
-buffer header spinlock, which would have to be taken anyway to decrement the
+small limit value) whenever the buffer is pinned.  (This requires only the
+buffer header spinlock, which would have to be taken anyway to increment the
 buffer reference count, so it's nearly free.)
 
 The "clock hand" is a buffer index, NextVictimBuffer, that moves circularly