doc: flow6 label should be numeric op, not bitmask

[Eric Long]

Hi,

In User's Guide Section 6.6 (Protocol :: Static) [0], IPv6 flowspec's 
flow label component is documented as:

 > label bitmask-match
 >
 >    Set a 20-bit bitmask for matching Flow Label field in packets (e.g.
 >    label 0x8e5/0x8e5).

However in RFC 8956 (Dissemination of Flow Specification Rules for IPv6) 
as well as earlier drafts it should be followed by numeric operators 
instead of bitmasks.

Source code seems to be correct [2], though.

p.s. Could BGP's "supported standards" replace RFC 5575 with 8955/8956?

Cheers,
Eric

[0]: https://bird.network.cz/?get_doc&v=30&f=bird-6.html#ss6.16
[1]: https://www.rfc-editor.org/rfc/rfc8956.html#name-type-13-flow-label-new
[2]: 
https://gitlab.nic.cz/labs/bird/-/blob/96dc63d602016c38f43677b2f95705aee9dc04e1/lib/flowspec.c#L482