Fix r3-gui.r3 download URL extraction

Fix a typo in the URL extraction for r3-gui.r3. Without that fix, I get the following error when starting from a clean download of RSOChat:

```
>> do %rsochat/rsochat.r3
Script: "Rebol Stack Overflow Chat Client" Version: 0.0.99 Date: 17-Jun-2013 19-
Jun-2013 21-Jun-2013 4-May-2014
logging event starting ..
logging event fetching prot-http.r3
logging event https://raw.githubusercontent.com/gchiu/Rebol3/master/protocols/pr
ot-http.r3
logging event prot-http.r3
Module: "REBOL 3 HTTP protocol scheme" Version: 0.1.45 Date: 27-Apr-2014
logging event fetching r3-gui.r3
** Script error: url has no value
** Where: switch unless unless foreach catch either either -apply- do
** Near: switch/default module [
    %r3-gui.r3 [
        test: body-...
```