docs: update upper limit for layers in a bundle

The max number of layers in a single tekton oci bundle is 20 (in the
code). This make sure the contract document reflects that.

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

docs/tekton-bundle-contracts.md

@@ -17,7 +17,7 @@ bundle reference.
 
 Each layer of the image must map 1:1 with a single Tekton resource (eg Task).
 
-*No more than 10* individual layers (Pipelines and/or Tasks) maybe placed in a single image.
+*No more than 20* individual layers (Pipelines and/or Tasks) maybe placed in a single image.
 
 Each layer must contain all of the following annotations: