rtp_engine.h: Update examples using ast_format_set.

This file includes some doxygen comments referencing
ast_format_set. This is an obsolete API that was
removed years back, but documentation was not fully
updated to reflect that. These examples are
updated to the current way of doing things
(using the format cache).

ASTERISK-30327 #close

Change-Id: I570f3b8007fa17ba470cc7117f44bfe7c555d2f7