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/mills/index.html

@@ -758,7 +758,7 @@ to queries about how I was spending my free time. Similarly, I apologise to
 those friends and coworkers who had to listen to me complain about obscure
 corner-case bugs in my code. The obsession is over, at least until next
 time…</p>
-<hr />
+<hr style="width:10%;text-align:left;margin-left:0">
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