[DOC] Tweak the doc for `Process.kill` signature

[koic]

Replacing ... with *pids seems to clarify the expected variadic arguments.

Note that the expected arguments are two or more with a signal and pids. That is, the method must have at least one pid, which cannot be omitted:

% ruby -e 'Process.kill(0)'
-e:1:in `kill': wrong number of arguments (given 1, expected 2+) (ArgumentError)
        from -e:1:in `<main>'