[PATCH] Make spelling consistent in waiteventset.c

Hi,

The four backend-specific paths in WaitEventSetWaitBlock()
(epoll, kqueue, poll, and Windows) had identical-purpose comments
with inconsistent spellings.

Two comments used "writable" while the other two used "writeable".

This patch standardizes the spelling to "writable" to match the
surrounding text in the file.

The WL_SOCKET_WRITEABLE macro is a public identifier and is left
unchanged.

Patch attached.