fix example bundle dockerfile

pre-fix this would copy all yaml files in `./manifests` into the _file_
`/manifests` and it would only keep the last file's contents.

Adding that trailing slash creates the `/manifests` directory and copies
all files as expected.

Signed-off-by: Josh Gwosdz <[email protected]>

Author: erdii

docs/design/operator-bundle.md

@@ -106,7 +106,7 @@ LABEL operators.operatorframework.io.bundle.package.v1=test-operator
 LABEL operators.operatorframework.io.bundle.channels.v1=beta,stable
 LABEL operators.operatorframework.io.bundle.channel.default.v1=stable
 
-ADD test/*.yaml /manifests
+ADD test/*.yaml /manifests/
 ADD test/metadata/annotations.yaml /metadata/annotations.yaml
 ```