Restore GOG reference in INSTALL.markdown

GOG.com resumed selling Descent 1 and Descent 2, albeit as separate SKUs
rather than the historical bundled SKU.  Restore the GOG reference in
INSTALL.markdown.

Reported-by: derhass <https://forum.dxx-rebirth.com/showthread.php?tid=1004>

INSTALL.markdown

@@ -195,4 +195,4 @@ For Windows and Linux, DXX-Rebirth installs only the main game binary.  The bina
 
 As a convenience, if **register\_install\_target=True**, **SConstruct** registers a pseudo-target named **install** which copies the compiled files to *DESTDIR*__/__*BINDIR*.  By default, **register\_install\_target=True**, *DESTDIR* is *empty*, and *BINDIR* is *PREFIX*__/bin__, which expands to **/usr/local/bin**.
 
-DXX-Rebirth [requires game data](http://www.dxx-rebirth.com/game-content/) to play.  The build system has no support for interacting with game data.  You can get [Descent 1 PC shareware data](http://www.dxx-rebirth.com/download/dxx/content/descent-pc-shareware.zip) and [Descent 2 PC demo data](http://www.dxx-rebirth.com/download/dxx/content/descent2-pc-demo.zip) from the DXX-Rebirth website.  Full game data is supported (and recommended), but is not freely available.
+DXX-Rebirth [requires game data](http://www.dxx-rebirth.com/game-content/) to play.  The build system has no support for interacting with game data.  You can get [Descent 1 PC shareware data](http://www.dxx-rebirth.com/download/dxx/content/descent-pc-shareware.zip) and [Descent 2 PC demo data](http://www.dxx-rebirth.com/download/dxx/content/descent2-pc-demo.zip) from the DXX-Rebirth website.  Full game data is supported (and recommended), but is not freely available.  You can [buy full Descent 1 game data](https://www.gog.com/game/descent) and/or [buy full Descent 2 game data](https://www.gog.com/game/descent_2) from GOG.com.  Historically, both Descent 1 and Descent 2 were sold as a single unit.  After a nearly two-year hiatus from sale, the games returned to GOG.com in November 2017 as separate units.  DXX-Rebirth contains engines for both games.  Each engine works for its respective game without the data from the other, so players who wish to purchase only one game may do so.