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`.

thanks-for-help.html

@@ -843,7 +843,7 @@ class="sourceCode diff"><code class="sourceCode diff"><span id="cb30-1"><a href=
 <span id="cb30-3"><a href="#cb30-3" aria-hidden="true" tabindex="-1"></a><span class="dt">@@ -7,13 +7,12 @@ if [[ $# -ne 1 ]]; then</span></span>
 <span id="cb30-4"><a href="#cb30-4" aria-hidden="true" tabindex="-1"></a>     exit 1</span>
 <span id="cb30-5"><a href="#cb30-5" aria-hidden="true" tabindex="-1"></a> fi</span>
-<span id="cb30-6"><a href="#cb30-6" aria-hidden="true" tabindex="-1"></a> </span>
+<span id="cb30-6"><a href="#cb30-6" aria-hidden="true" tabindex="-1"></a></span>
 <span id="cb30-7"><a href="#cb30-7" aria-hidden="true" tabindex="-1"></a> # run/compile it</span>
 <span id="cb30-8"><a href="#cb30-8" aria-hidden="true" tabindex="-1"></a> rm -f ouroboros.c x1 x</span>
 <span id="cb30-9"><a href="#cb30-9" aria-hidden="true" tabindex="-1"></a><span class="st">-ex - &lt;&lt;EOF</span></span>