doc: sync help description for --trace with man for qemu.1

[s/descriprion/description/ in commit message as suggested by Eric Blake
<eblake@redhat.com>.
--Stefan]
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 1466174654-30130-2-git-send-email-den@openvz.org
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qemu-options.hx

@@ -3669,7 +3669,7 @@ DEF("trace", HAS_ARG, QEMU_OPTION_trace,
 STEXI
 HXCOMM This line is not accurate, as some sub-options are backend-specific but
 HXCOMM HX does not support conditional compilation of text.
-@item -trace [events=@var{file}][,file=@var{file}]
+@item -trace [[enable=]@var{pattern}][,events=@var{file}][,file=@var{file}]
 @findex -trace
 
 Specify tracing options.