fix even more broken markdown links

Added `bin/find-missing-links.sh` tool to check for
markdown links to missing local files.

Regarding the missing link tool:

```
    Here be extended regular expression magic dragons! :-)
```

Added `make find_missing_links` to the top level Makefile.

Added `make find_missing_links` to `make www` and `make quick_www`.

Ran `make www` to perform the above.

TODO: Fix markdown links to missing local files.

1993/rince/README.md

@@ -74,7 +74,7 @@ Use `rince.alt` or `rince.alt2` as you would `rince` above.
 The author has provided several CABBAGE files for your amusement:
 
 - [rince.data1](%%REPO_URL%%/1993/rince/rince.data1) plays
-[Breakout](https://en.wikipedia.org/wiki/Breakout_(video_game).  Keys are `,`
+[Breakout](https://en.wikipedia.org/wiki/Breakout_(video_game)).  Keys are `,`
 for left, `/` for right, and `.` for 'fire the ball'.
 
 - [rince.data2](%%REPO_URL%%/1993/rince/rince.data2) plays the old [James Bonfield](../../authors.html#James_Bonfield)'s