fix HTML problems associated with leading ---'s

Please do not use "**---**" in markdown lines to separate sections.

This causes `pandoc(1)` to generate "**\<hr />**".

The "**\<hr />**" has no effect in standard HTML 5 and interacts
badly with unquoted attribute values.

If a horizontal line is really needed, use "**\<hr>**".

If a short line is needed, use: "**\<hr
style="width:10%;text-align:left;margin-left:0">**"

2018/burton2/README.md

@@ -341,7 +341,11 @@ The following reserved word files are included:
 
 NB: The keyword file used in this code is derived from the list in [iocccsize.c](../../2018/iocccsize.c),
 which is neither complete (`#define`, `#ifndef`, `#undef` are missing
--- yes, Virginia knows about `#define` omitted on purpose),
+
+```
+yes, Virginia knows about `#define` omitted on purpose
+``
+
 nor correct (many more are added: `I`, `true`, `bool`, `compl`, ...):
 
 ``` <!---sh-->