sway-output.5: fix variable name
sway-output.5: fix variable name

__foo__ is verbatim in scdoc, but we want the variable foo in italic,
i.e. _foo_.

1ace/sway,1ace/sway,ascent12/sway,ascent12/sway,ascent12/sway,SirCmpwn/sway,1ace/sway