doc/examples/muxing: Always use swr, simplifies code slightly

Idea-from: 56f98e340fca894a76d1ddbe33118b8d8c4db34a
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/muxing: mark correct frame as writeable

Bug found from comparing 56f98e340fca894a76d1ddbe33118b8d8c4db34a to HEAD
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>