[Doc] Update regex engine to Onigumo in doc/extension.* [ci skip]

regex.c has been removed in 8e65234086

doc/extension.ja.rdoc

@@ -1108,9 +1108,8 @@ lex.c         :: 自動生成
     -> opt*.inc            : 自動生成
     -> vm.inc              : 自動生成
 
-=== 正規表現エンジン (鬼車)
+=== 正規表現エンジン (鬼雲)
 
-  regex.c
   regcomp.c
   regenc.c
   regerror.c

doc/extension.rdoc

@@ -1076,9 +1076,8 @@ lex.c         :: automatically generated from keywords
     -> opt*.inc            : automatically generated
     -> vm.inc              : automatically generated
 
-=== Regular Expression Engine (Oniguruma)
+=== Regular Expression Engine (Onigumo)
 
-  regex.c
   regcomp.c
   regenc.c
   regerror.c