Doc: silence A4 PDF build warnings.

Commit 0fbceae84 put a "&zwsp;" in almost but not quite the correct
place to avoid "The contents of fo:block line 1 exceed the available
area" warnings.  Per buildfarm.

doc/src/sgml/perform.sgml

@@ -840,7 +840,7 @@ EXPLAIN ANALYZE SELECT * FROM tenk1 WHERE thousand IN (1, 500, 700, 999);
 <screen>
 EXPLAIN ANALYZE SELECT * FROM tenk1 WHERE thousand IN (1, 2, 3, 4);
                                                             QUERY PLAN
---------------------------------------------------------------------&zwsp;--------------------------------------------------------------
+-------------------------------------------------------------------&zwsp;---------------------------------------------------------------
  Bitmap Heap Scan on tenk1  (cost=9.45..73.44 rows=40 width=244) (actual time=0.009..0.019 rows=40.00 loops=1)
    Recheck Cond: (thousand = ANY ('{1,2,3,4}'::integer[]))
    Heap Blocks: exact=38