docs/bpf: update out-of-date doc in BPF flow dissector

Commit a5e2151f ("net/ipv6: SKB symmetric hash should incorporate
transport ports") removed the use of FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL
in __skb_get_hash_symmetric(), making the doc out-of-date.
Signed-off-by: Quan Tian <qtian@vmware.com>
Link: https://lore.kernel.org/r/20230911152353.8280-1-qtian@vmware.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

Documentation/bpf/prog_flow_dissector.rst

@@ -113,7 +113,7 @@ Flags
   used by ``eth_get_headlen`` to estimate length of all headers for GRO.
 * ``BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL`` - tells BPF flow dissector to
   stop parsing as soon as it reaches IPv6 flow label; used by
-  ``___skb_get_hash`` and ``__skb_get_hash_symmetric`` to get flow hash.
+  ``___skb_get_hash`` to get flow hash.
 * ``BPF_FLOW_DISSECTOR_F_STOP_AT_ENCAP`` - tells BPF flow dissector to stop
   parsing as soon as it reaches encapsulated headers; used by routing
   infrastructure.