Proof-reading for documentation.

Somebody accidentally a word.  Back-patch to 9.6.

Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/20180816195431.GA23707%40telsasoft.com

doc/src/sgml/parallel.sgml

@@ -541,7 +541,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     unsafe, because this would require predicting every operation which the
     function could possibly perform.  In general, this is equivalent to the
     Halting Problem and therefore impossible.  Even for simple functions
-    where it conceivably be done, we do not try, since this would be expensive
+    where it could conceivably be done, we do not try, since this would be expensive
     and error-prone.  Instead, all user-defined functions are assumed to
     be parallel unsafe unless otherwise marked.  When using
     <xref linkend="sql-createfunction"/> or