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
 
 ```