Fix assembly reference in template to drop unneeded ".dll" extension

MonoDevelop's assembly reference resolver doesn't work if you have "Gac"
references with file extensions. Drop the .dll, and MonoDevelop will find
Tao from a suitable location such as the GAC or a pkg-config package.

ProjectTemplates/MonoDevelop.MonoGame.2.5/templates/MonoGameLinuxProject.xpt.xml

@@ -26,7 +26,7 @@
 				<Reference type="Gac" refto="OpenTK" />
 				<Reference type="Gac" refto="MonoGame.Framework.Linux" />
 				<Reference type="Gac" refto="Lidgren.Network" />
-				<Reference type="Gac" refto="Tao.Sdl.dll" />
+				<Reference type="Gac" refto="Tao.Sdl" />
 			</References>
 			<Files>
 				<File name="Game1.cs" src="MonoGameApplication/Game1.cs" />