remove trailing blanks in markdown files

We used this command to find markdown files with trailing whitespace.

```sh
find . -name '*.md' -print0 | xargs -0 grep -E -l '[[:space:]][[:space:]]*$'
```

After removal, we did a `make www`.

2020/giles/README.md

@@ -37,7 +37,7 @@ will be converted to the DTMF sound!
 ./try.sh
 ./prog < pi.wav
 
-# See 
+# See
 ./prog 867-5309 > jenny.wav
 
 ```

2020/giles/index.html

@@ -160,7 +160,7 @@ the DTMF sound!</p>
 <div class="sourceCode" id="cb4"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./try.sh</span></span>
 <span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a><span class="ex">./prog</span> <span class="op">&lt;</span> pi.wav</span>
 <span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a></span>
-<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a><span class="co"># See </span></span>
+<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a><span class="co"># See</span></span>
 <span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a><span class="ex">./prog</span> 867-5309 <span class="op">&gt;</span> jenny.wav</span></code></pre></div>
 <p>If the script <a href="try.sh">try.sh</a> did not play a WAV file, it
 means you do not have SoX installed; see <a