add missing underscore to man page and example nf_mark ematch

add missing underscore to man page and example nf_mark ematch

The man page and the "fail" example are missing an underscore in the
nf_mark ematch.

eg.

tc filter add dev eth0 parent ffff:  basic match 'meta(nfmark gt 24)'
classid 2:4

meta: unknown meta id

... >>meta(nfmark gt 24)<< ...
... meta(>>nfmark<< gt 24)...
Usage: meta(OBJECT { eq | lt | gt } OBJECT)
where: OBJECT  := { META_ID | VALUE }
        META_ID := id [ shift SHIFT ] [ mask MASK ]

Example: meta(nfmark gt 24)
          meta(indev shift 1 eq "ppp")
          meta(tcindex mask 0xf0 eq 0xf0)

For a list of meta identifiers, use meta(list).
Illegal "ematch"

meta(list) does correctly show nf_mark and the above test works with
nf_mark.

Signed-off-by: Andy Furniss adf.lists@gmail.com