doc: adjust libpq wording to be neither/nor

Reported-by:  <postgresql@cohi.at>

Discussion: https://postgr.es/m/155419437926.737.10876947446993402227@wrigleys.postgresql.org

Backpatch-through: 9.4

doc/src/sgml/libpq.sgml

@@ -3578,7 +3578,7 @@ char *PQescapeLiteral(PGconn *conn, const char *str, size_t length);
      </tip>
 
      <para>
-      Note that it is not necessary nor correct to do escaping when a data
+      Note that it is neither necessary nor correct to do escaping when a data
       value is passed as a separate parameter in <function>PQexecParams</> or
       its sibling routines.
      </para>