Fix typo in a comment in trie_update_elem().

Signed-off-by: Leon Huayra <hffilwlqm@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211229144422.70339-1-hffilwlqm@gmail.com