AsyncResult: Fix grammar error, <tt>ify enumerator
AsyncResult: Fix grammar error, <tt>ify enumerator

This was not a grammar error before C++ification! But let’s fix it
anyway, and give the enumeration name some nice formatting as well.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm