unix: fix a comment typo in signal.c
unix: fix a comment typo in signal.c

PR-URL: https://github.com/libuv/libuv/pull/2318
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>
Reviewed-By: Сковорода Никита Андреевич <767c993c25b04139f9abc27a1bb4597f429afab3@gmail.com>
Reviewed-By: Vladimír Čunát <7a38d8cbd20d9932ba948efaa364bb62651d5ad4@cunat.cz>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso