🐧 Use %F instead of %U for file variable

The desktop entry specification states that %U is for URLs and %F is for
files. Since atom doesn't support URLs, we should use %F. Fixes #2320.

resources/linux/atom.desktop.in

@@ -2,7 +2,7 @@
 Name=<%= appName %>
 Comment=<%= description %>
 GenericName=Text Editor
-Exec=<%= installDir %>/share/<%= appFileName %>/atom %U
+Exec=<%= installDir %>/share/<%= appFileName %>/atom %F
 Icon=<%= iconPath %>
 Type=Application
 StartupNotify=true