docs: use os.pardir

I'm not really sure if this ever matters in real-life, but os.pardir
exists and we should probably use it instead of hard-coding it to '..'.

Fixes: 67485efd65 ("docs: prepare for hawkmoth")
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11494>

docs/conf.py

@@ -218,7 +218,7 @@ graphviz_output_format = 'svg'
 
 # -- Options for hawkmoth -------------------------------------------------
 
-hawkmoth_root = os.path.abspath('..')
+hawkmoth_root = os.path.abspath(os.pardir)
 hawkmoth_clang = [
   '-Idocs/header-stubs/',
   '-Iinclude/',