Depend on $PATH to find executable in atom.desktop

With the atom atom executable now located in /usr/bin instead
of /usr/local/bin, it should always be available as part of the
system PATH. Thus hardcoding the filepath is not needed. Also, this
increase the flexibility of relocating the rpm at installation time
(not just build time) since the user or sys admin need only make
sure that the atom executable is in the system PATH and the
atom.desktop file will work correctly.

resources/linux/atom.desktop.in

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