Fix GitHub case in BUILD

Replace "Github" with "GitHub".

PR #3218 <https://github.com/Genymobile/scrcpy/pull/3218>

Signed-off-by: Romain Vimont <rom@rom1v.com>

BUILD.md

@@ -46,7 +46,7 @@ sudo ninja -Cbuild-auto uninstall
 ### `master`
 
 The `master` branch concerns the latest release, and is the home page of the
-project on Github.
+project on GitHub.
 
 
 ### `dev`