Fix a comment [ci skip]

Author: nobu

tool/m4/ruby_append_option.m4

@@ -4,6 +4,6 @@ AC_DEFUN([RUBY_APPEND_OPTION],
 	AS_CASE([" [$]{$1-} "],
 	[*" $2 "*], [], ['  '], [ $1="$2"], [ $1="[$]$1 $2"])])dnl
 AC_DEFUN([RUBY_PREPEND_OPTION],
-	[# RUBY_APPEND_OPTION($1)
+	[# RUBY_PREPEND_OPTION($1)
 	AS_CASE([" [$]{$1-} "],
 	[*" $2 "*], [], ['  '], [ $1="$2"], [ $1="$2 [$]$1"])])dnl