Fix typo in 09_parallel_join.md
I was reading online, and spotted a minor typo - the 'el' was omitted in 'parallelization'. Add it back.
@@ -7,7 +7,7 @@ to the same resource where at least one of them needs write access to it).
[c3]: ./03_dispatcher.html
-## Basic parallization
+## Basic parallelization
What isn't automatically parallelized by Specs are
the joins made within a single system:
