man: fix formatting of underscores

In org-mode, underscores are used to represent subscripts.  Thus, `a_b`
will be transcoded to `a\d\s-2b\s+2\u`.  Configure org-mode only to do
this for `a_{b}`.

Signed-off-by: Jeremy Sowden <azazel@debian.org>

man/meson.build

@@ -81,6 +81,7 @@ foreach src : man_orgs
   expr_tmpl = ''.join([
     '(progn',
     '  (require \'ox-man)',
+    '  (setq org-export-with-sub-superscripts \'{})',
     '  (org-export-to-file \'man "@0@"))'])
   expr = expr_tmpl.format(org.substring(0,-4))
   sectiondir = join_paths(mandir, 'man' + section)