Swap "single-process" and "multi-process" in process-dies-while-detaching.exp

Swap "single-process" and "multi-process" in process-dies-while-detaching.exp

"single-process" and "multi-process" are used in the test message of
process-dies-while-detaching.exp, but they are misplaced due to

    set mode [expr {$multi_process ? "single-process" : "multi-process"}]

This patch is to swap them.

gdb/testsuite:

2016-08-01  Yao Qi  <yao.qi@linaro.org>

	* gdb.threads/process-dies-while-detaching.exp (do_test): Set
	variable mode to "multi-process" if $multi_process is 1, otherwise
	set it to "single-process".