docs: make process_kill signum arg optional

[Bilal2453]

In Libuv, uv_process_kill takes a process and a signum, where the signum is required. In Luv although, in luv_parse_signal, if the input is not an integer and not a string, the default value of SIGTERM is used.

Effectively making signum an optional argument with the default string value sigterm.

[squeek502]

The relevant code:

https://github.com/luvit/luv/blob/9e3c2320149884e5d809e9391f8580bb5f67bcdf/src/process.c#L302-L310

[Bilal2453]

Seems like I forgot uv.kill in this PR.