Android Training: Add missing word

The description of when to use an IntentService in create-service.jd is
missing a word, which makes it difficult to follow. The cruical phrase
is that an IntentService "is the preferred way to *perform* simple
background operations".

Change-Id: I13e4338cd2bad968d05b945ea398e2332f307850
(cherry picked from commit eb26cfa4)