website: Fix wrong markdown in installation page
The extra '=======' creates an unwanted heading in HTML.
@@ -156,4 +156,3 @@ described above.
Please take the known limitations for [layer2](https://metallb.universe.tf/concepts/layer2/#limitations)
and [bgp](https://metallb.universe.tf/concepts/bgp/#limitations) into account when performing an
upgrade.
-=======
