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.

index b9bf70c..4726673 100644
@@ -113,7 +113,7 @@ The original interface (i.e. the device the packet arrived on) is treated as the
 
 # 'cmp(u16 at 3 layer 2 mask 0xff00 gt 20)'
 
-# 'meta(nfmark gt 24)' and 'meta(tcindex mask 0xf0 eq 0xf0)'
+# 'meta(nf_mark gt 24)' and 'meta(tcindex mask 0xf0 eq 0xf0)'
 
 # 'nbyte("ababa" at 12 layer 1)'
 

index fad6b12..b64f333 100644
@@ -32,7 +32,7 @@ static void meta_print_usage(FILE *fd)
            "where: OBJECT  := { META_ID | VALUE }\n" \
            "       META_ID := id [ shift SHIFT ] [ mask MASK ]\n" \
            "\n" \
-           "Example: meta(nfmark gt 24)\n" \
+           "Example: meta(nf_mark gt 24)\n" \
            "         meta(indev shift 1 eq \"ppp\")\n" \
            "         meta(tcindex mask 0xf0 eq 0xf0)\n" \
            "\n" \