fix typo in training article: "SMP Primer for Android"

See: http://code.google.com/p/android/issues/detail?id=60136

On page http://developer.android.com/training/articles/smp.html, do a page search for "C+++", that should probably be "C++"

Change-Id: I1aa5ff2ab116fa3131fbd64f6ee203aac39fe8bd