docs: Explicitly set UFT-8 encoding in po4a

Author: rdmark

doc/po4a.cfg

@@ -1,7 +1,7 @@
 [po4a_langs] ja
 [po4a_paths] po/$master.pot $lang:po/$master.$lang.po
 
-[options] markdown
+[options] markdown --master-charset UTF-8
 
 [type: text] manpages/man1/ad.1.md $lang:translated/$lang/ad.1.md
 [type: text] manpages/man1/addump.1.md $lang:translated/$lang/addump.1.md