doc: Add missing semicolon in example

One of the examples on the SELECT page was missing a semicolon from
a listing which has the look and feel of being a psql session. This
adds the missing semicolon and also removes the newline between the
query and results to match the other examples nearby.

Backpatch to all supported branches to avoid backpatching issues on
this page.

Reported-by: tim.needham2@gmail.com
Discussion: https://postgr.es/m/169965004097.225187.12941375915673151540@wrigleys.postgresql.org
Backpatch-through: v12

doc/src/sgml/ref/select.sgml

@@ -1918,8 +1918,7 @@ WITH t AS (
   )
 SELECT * FROM t
 UNION ALL
-SELECT * FROM t
-
+SELECT * FROM t;
          x          
 --------------------
   0.534150459803641