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`.

1992/buzzard.2/buzzard.2.README.md

@@ -3,7 +3,7 @@
 [third](third)				    implementation of THIRD in FIRST\
 [help.th](help.th)			    online listing of THIRD primitives\
 
-	
+
 # FIRST demos:
 
 Use  `./first < <demo>`.