contrib/launchd: fix xml syntax error

contrib/launchd: fix xml syntax error

The current plist xml gets error "DOCTYPE improperly terminated" with xml
syntax checker[1]. The example in apple doc[2] also doesn't have semicolon
at the end of DOCTYPE line.

[1] https://www.w3schools.com/xml/xml_validator.asp
[2] https://opensource.apple.com/source/launchd/launchd-257/launchd/doc/HOWTO.html

Fixes: b30e74b5956a ("wg-quick: darwin: support being called from launchd")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>