doc, af_xdp: Fix bind flags option typo

Fix XDP_ZERO_COPY flag typo since it is actually named XDP_ZEROCOPY
instead as per if_xdp.h uapi header.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/1656fdf94704e9e735df0f8b97667d8f26dd098b.1625550240.git.baruch@tkos.co.il

Documentation/networking/af_xdp.rst

@@ -243,16 +243,16 @@ Configuration Flags and Socket Options
 These are the various configuration flags that can be used to control
 and monitor the behavior of AF_XDP sockets.
 
-XDP_COPY and XDP_ZERO_COPY bind flags
--------------------------------------
+XDP_COPY and XDP_ZEROCOPY bind flags
+------------------------------------
 
 When you bind to a socket, the kernel will first try to use zero-copy
 copy. If zero-copy is not supported, it will fall back on using copy
 mode, i.e. copying all packets out to user space. But if you would
 like to force a certain mode, you can use the following flags. If you
 pass the XDP_COPY flag to the bind call, the kernel will force the
 socket into copy mode. If it cannot use copy mode, the bind call will
-fail with an error. Conversely, the XDP_ZERO_COPY flag will force the
+fail with an error. Conversely, the XDP_ZEROCOPY flag will force the
 socket into zero-copy mode or fail.
 
 XDP_SHARED_UMEM bind flag