fix fenced code block syntax for comprehensibility

ikiwiki.info does not support ``` as a fence for code blocks, only ~~~,
and it only works at the beginning of a new paragraph.

doc/forum/Cannot_run_ikiwiki_due_to_Encode.pm/comment_1_3be380ae32e9e6f65d47975827950050._comment

@@ -5,11 +5,12 @@
  date="2015-02-25T05:43:26Z"
  content="""
 I fixed it by adding
-```
+
+~~~
 sub decode($$;$) {
     my ( $name, $octets, $check ) = @_;
     return undef unless defined $octets;
     return $octets if Encode::is_utf8($octets);  # add this check
 	$octets .= '';
-```
+~~~
 """]]