Fix broken link to github repo

Fixes KhronosGroup#1506

Author: SaschaWillems

antora/modules/ROOT/nav.adoc

@@ -135,4 +135,4 @@
 ** xref:docs/build.adoc[Build guide]
 ** xref:docs/memory_limits.adoc[Memory limits]
 ** xref:docs/misc.adoc[Miscellaneous]
-* link:https://github.com/KhronosGroup/Vulkan-Samples[GitHub Repository, window=_blank
+* link:https://github.com/KhronosGroup/Vulkan-Samples[GitHub Repository, window=_blank]