[gnome-boxes.wiki] Replace references of flathub with gnome-nightly (for "master" sdk/runtime)

[Felipe Borges <felipeborges src gnome org>]

commit 992dc77f0781bef5f0644b130a0c342e819abb03
Author: Felipe Borges <felipeborges gnome org>
Date:   Fri Jan 14 08:41:31 2022 +0000

    Replace references of flathub with gnome-nightly (for "master" sdk/runtime)

 Building-instructions.md | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)
---
diff --git a/Building-instructions.md b/Building-instructions.md
index 86f7f48..5ab99d5 100644
--- a/Building-instructions.md
+++ b/Building-instructions.md
@@ -29,11 +29,13 @@ Where `<build-dir>` is the path to the directory that Boxes will be built into.
 Runtimes and SDKs are usually already present in your system if you have installed Flatpak apps or have 
attempted to develop another application with Flatpak, but if you need to manually install them, you can run:
 
 ```
-$ flatpak install flathub org.gnome.Platform
-$ flatpak install flathub org.gnome.Sdk
+$ flatpak install gnome-nightly org.gnome.Platform
+$ flatpak install gnome-nightly org.gnome.Sdk
 ```
 
-Where [flathub](https://flathub.org) is the repository where the org.gnome.Platform runtime and the 
org.gnome.Sdk SDK are hosted.
+Where [gnome-nightly](https://wiki.gnome.org/Apps/Nightly) is the repository where the unstable (master) 
org.gnome.Platform runtime and the org.gnome.Sdk SDK are hosted.
+
+Visit https://wiki.gnome.org/Apps/Nightly to learn how to add the `gnome-nightly` repository.
 
 ## Building against your host system