Doc: fix typo in hash index documentation

Plus a similar fix to the README.

Backpatch as far back as the sgml issue exists.  The README issue does
exist in v14, but that seems unlikely to harm anyone.

Author: David Geier <geidav.pg@gmail.com>
Discussion: https://postgr.es/m/ed3db7ea-55b4-4809-86af-81ad3bb2c7d3@gmail.com
Backpatch-through: 15

doc/src/sgml/hash.sgml

@@ -125,11 +125,10 @@
   Both scanning the index and inserting tuples require locating the bucket
   where a given tuple ought to be located. To do this, we need the bucket
   count, highmask, and lowmask from the metapage; however, it's undesirable
-  for performance reasons to have to have to lock and pin the metapage for
+  for performance reasons to have to lock and pin the metapage for every such
-  every such operation. Instead, we retain a cached copy of the metapage
+  operation. Instead, we retain a cached copy of the metapage in each
-  in each backend's relcache entry. This will produce the correct bucket
+  backend's relcache entry. This will produce the correct bucket mapping as
-  mapping as long as the target bucket hasn't been split since the last
+  long as the target bucket hasn't been split since the last cache refresh.
-  cache refresh.
  </para>
 
  <para>

src/backend/access/hash/README

@@ -171,11 +171,10 @@ Metapage Caching
 Both scanning the index and inserting tuples require locating the bucket
 where a given tuple ought to be located.  To do this, we need the bucket
 count, highmask, and lowmask from the metapage; however, it's undesirable
-for performance reasons to have to have to lock and pin the metapage for
+for performance reasons to have to lock and pin the metapage for every such
-every such operation.  Instead, we retain a cached copy of the metapage
+operation.  Instead, we retain a cached copy of the metapage in each backend's
-in each backend's relcache entry.  This will produce the correct
+relcache entry.  This will produce the correct bucket mapping as long as the
-bucket mapping as long as the target bucket hasn't been split since the
+target bucket hasn't been split since the last cache refresh.
-last cache refresh.
 
 To guard against the possibility that such a split has occurred, the
 primary page of each bucket chain stores the number of buckets that