Fix doc for full text search distance operator.

Commit 028350f6 changed its behavior from "at most" to "exactly", but
forgot to update the documentation. Backpatch to 9.6.

Patch by Justin Pryzby, per Yaroslav Schekin's report.

Discussion: https://www.postgresql.org/message-id/20201005191922.GE17626%40telsasoft.com

doc/src/sgml/textsearch.sgml

@@ -1645,7 +1645,7 @@ SELECT to_tsquery('fat') <-> to_tsquery('cat | rat');
      <listitem>
       <para>
        Returns a query that searches for a match to the first given query
-       followed by a match to the second given query at a distance of at
+       followed by a match to the second given query at a distance of exactly
        <replaceable>distance</replaceable> lexemes, using
        the <literal>&lt;<replaceable>N</replaceable>&gt;</literal>
        <type>tsquery</type> operator.  For example: