bpf, docs: Fix __uninit kfunc doc section

In commit d96d937 ("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: d96d937 ("bpf: Add __uninit kfunc annotation")
Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230301194910.602738-2-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/kfuncs.rst

@@ -101,7 +101,7 @@ size parameter, and the value of the constant matters for program safety, __k
 suffix should be used.
 
 2.2.2 __uninit Annotation
---------------------
+-------------------------
 
 This annotation is used to indicate that the argument will be treated as
 uninitialized.