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`.

1987/README.md

@@ -13,7 +13,7 @@ The practice of posting a preliminary set of rules for the next year was started
 this year.
 
 [Rules](rules.txt) and results were posted to comp.lang.c and comp.unix.wizards
-with an announcement in news.announce.important.  
+with an announcement in news.announce.important.
 [Micro/Systems Journal](https://www.vintage-computer.com/publications.php?microsystemsjournal)
 published the 1987 winning entries.  [Mark R.
 Horton](https://www.amazon.com/stores/Mark-R.-Horton/author/B001HPGRB8) included

1987/index.html

@@ -138,8 +138,7 @@ abuse was unusual. The mailboxes for the contest were moved from nsc to
 amdahl. The practice of posting a preliminary set of rules for the next
 year was started this year.</p>
 <p><a href="rules.txt">Rules</a> and results were posted to comp.lang.c
-and comp.unix.wizards with an announcement in
-news.announce.important.<br />
+and comp.unix.wizards with an announcement in news.announce.important.
 <a
 href="https://www.vintage-computer.com/publications.php?microsystemsjournal">Micro/Systems
 Journal</a> published the 1987 winning entries. <a