[alsa-devel] speaker-test man page should mention the proper way to exit

The speaker-test man page should mention the proper way to exit speaker-test.

^C gives

Write error: -4,Interrupted system call xrun_recovery failed: -4,Interrupted system call Transfer failed: Interrupted system call

except when run with -c 2.

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802935