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">**"

2019/duble/index.html

@@ -171,7 +171,7 @@ See <a href="#alternate-code">Alternate code</a> below if you are curious.</p>
 <p>Alternatively, if you want to change the size, do something like:</p>
 <pre><code>    make clobber LINES=20 COLUMNS=20 all</code></pre>
 <p>The author provided us with the following table for use:</p>
-<hr />
+<hr style="width:10%;text-align:left;margin-left:0">
 <ul>
 <li><strong>Arrow keys</strong>: move cursor.</li>
 <li><code>p</code> key: enter <em>pen</em> mode (type <strong>p</strong> again to return to the default <em>move</em>
@@ -186,7 +186,7 @@ the editor.</p>
 <p>After editing the file and quitting, try:</p>
 <pre><code>    /tmp/drawing</code></pre>
 <p>i.e. execute the file.</p>
-<hr />
+<hr style="width:10%;text-align:left;margin-left:0">
 <p>WARNING: if the file is deleted it might lock any session still in use. These
 will have to be killed from another shell session or by closing the terminal tab
 (in other words don’t do this from the console!). If the file cannot be opened