Fix rst rendering

The file extention claims rst format, but it's plain text
as such, the online documentation doesn't render properly

Change-Id: I24accb45ab8c7803a25f2642ce1b2d479d5a6e9c

elements/package-installs/README.rst

@@ -3,26 +3,28 @@ uninstalling packages for an image build. This is done by creating a
 package-installs.yaml or package-installs.json file in the element directory.
 
 
-example package-installs.yaml:
-libxml2:
-grub2:
+example package-installs.yaml::
+
+ libxml2:
+ grub2:
    phase: pre-install.d
-networkmanager:
+ networkmanager:
    uninstall: True
 
-example package-installs.json:
-{
+example package-installs.json::
+
+    {
     "libxml2": null,
     "grub2": {"phase": "pre-install.d"},
     "networkmanager": {"uninstall": true}
-}
+    }
 
 
 Setting phase or uninstall properties for a package overrides the following
-default values:
+default values::
 
-phase: install.d
-uninstall: False
+    phase: install.d
+    uninstall: False
 
 
 DEPRECATED: Adding a file under your elements pre-install.d, install.d, or