bpf, docs: Fix __uninit kfunc doc section

In commit d96d937d7c5c ("bpf: Add __uninit kfunc annotation"), the
__uninit kfunc annotation was documented in kfuncs.rst. You have to
fully underline a section in rst, or the build will issue a warning that
the title underline is too short:

./Documentation/bpf/kfuncs.rst:104: WARNING: Title underline too short.

2.2.2 __uninit Annotation
--------------------

This patch fixes that title underline.

Fixes: d96d937d7c5c ("bpf: Add __uninit kfunc annotation")
Signed-off-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

Documentation/bpf/kfuncs.rst

···
 suffix should be used.
 
 2.2.2 __uninit Annotation
---------------------
+-------------------------
 
 This annotation is used to indicate that the argument will be treated as
 uninitialized.