lib/posix-fdtab: Fix docstring of `uk_fdtab_open`

lib/posix-fdtab: Fix docstring of `uk_fdtab_open`

Commit 5050e9f11e57 ("lib/posix-fd: Move heap-alloc ofiles out of fdtab")
did `s/uk_fdtab_ret/uk_ofile_release` across the codebase but forgot to
update the comment for `uk_fdtab_open`. Do so now, indicating to use
`uk_ofile_release` instead.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1619