bpf_doc: Fix build error with older python versions

The ability to subscript match result as an array is only available
since python 3.6. Existing code in bpf_doc uses the older group()
interface but commit 8a76145a adds code using the new interface.

Use the old interface consistently to avoid build error on older
distributions like the below:

+ make -j48 -s -C /dev/shm/kbuild/linux.33946/current ARCH=powerpc HOSTCC=gcc CROSS_COMPILE=powerpc64-suse-linux- clean
TypeError: '_sre.SRE_Match' object is not subscriptable

Fixes: 8a76145a ("bpf: explicitly define BPF_FUNC_xxx integer values")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20230109113442.20946-1-msuchanek@suse.de

scripts/bpf_doc.py

@@ -271,7 +271,7 @@ class HeaderParser(object):
             if capture:
                 fn_defines_str += self.line
                 helper_name = capture.expand(r'bpf_\1')
-                self.helper_enum_vals[helper_name] = int(capture[2])
+                self.helper_enum_vals[helper_name] = int(capture.group(2))
                 self.helper_enum_pos[helper_name] = i
                 i += 1
             else: