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`.

1998/schweikh3/README.md

@@ -102,7 +102,7 @@ Otherwise:
 
 ```sh
 ./samefile
-``` 
+```
 
 #### DESCRIPTION