Rename *.text *.md

As luck would have it the only entries where this applied were my own.
The links/references to these files have been changed in the files.

An unfortunate problem is that in 2020/ferguson1 there are several
*.markdown files. I will have to go through these and figure out which
ones should be changed to .md and which ones can be removed. This can
happen later but in the meantime the references are to .md. I believe
the only difference is that the .md has the footer added by the judges.
This was a workaround with the fact that I wasn't sure (and still am not
entirely sure) the generation of the html files will work. It might be
that the .markdown files have the footer and the .md files do not but
this can be dealt with later anyway.

2020/ferguson1/troubleshooting.html

@@ -107,7 +107,7 @@ e.g. when the snake is growing.</p>
 <p>  Sometimes also it might appear that the movement is delayed but there are
   numerous causes for this some of which are a busy computer but ultimately
   it&rsquo;ll catch up; just give it the time it needs.</p></li>
-<li><p>BOTTOM LINE: If you want more information then see the <a href="bugs.markdown">bugs.markdown</a> file.</p></li>
+<li><p>BOTTOM LINE: If you want more information then see the <a href="bugs.md">bugs.md</a> file.</p></li>
 </ul>
 
 
@@ -323,7 +323,7 @@ so that&rsquo;s probably why; in CentOS I did have to install it specially.</p>
   eat too many bytes in the entry (I cite the functions in the HACKING file).</p>
 
 <p>  I discuss colours and terminals in much more detail in
-  <a href="terminals.markdown">terminals.markdown</a> (<a href="terminals.html">terminals.html</a>).</p></li>
+  <a href="terminals.md">terminals.md</a> (<a href="terminals.html">terminals.html</a>).</p></li>
 </ul>
 
 
@@ -423,7 +423,7 @@ example</p>
 0.</p>
 
 <p>I discuss the signed/unsigned issue to do with curses and sizes in C being
-unsigned in <a href="bugs.markdown">bugs.markdown</a> but probably there isn&rsquo;t anything more to say here.</p>
+unsigned in <a href="bugs.md">bugs.md</a> but probably there isn&rsquo;t anything more to say here.</p>
 
 <h2><a name="errors" href="#toc">Errors and error reporting</a></h2>