capabilities.7: srcfix: Removed FIXME

capabilities.7: srcfix: Removed FIXME

No credential match of file UID and namespace creator UID
is needed to create a v3 security extended attribute.

Verified by experiment using my userns_child_exec.c and
show_creds.c programs (available on http://man7.org/tlpi/code):

    $ sudo setcap cap_setuid,cap_dac_override=pe \
            ./userns_child_exec
    $ ./userns_child_exec -U -r setcap cap_kill=pe show_creds
    $ ./userns_child_exec -U -M '0 1000 10' -G '0 1000 1' \
            -s 1 ./show_creds
    eUID = 1;  eGID = 0;  capabilities: = cap_kill+ep

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>