Change apply to use File (-f) instead of Kustomize (-k) (#5433)

Kustomize expects a directory containing a `kustomization.yaml` file as a parameter.  Trying to fetch the raw `bundle.yaml` does'nt work to install the CRDs.

With the -f option it works

Signed-off-by: Sébastien Lemay <smellems@gmail.com>

docs/introduction/getting-started.md

@@ -17,7 +17,7 @@ The default install options will automatically install and manage the CRDs as pa
 
 You can install those CRDs outside of `helm` using:
 ```bash
-kubectl apply -k "https://raw.githubusercontent.com/external-secrets/external-secrets/<replace_with_your_version>/deploy/crds/bundle.yaml" --server-side
+kubectl apply -f "https://raw.githubusercontent.com/external-secrets/external-secrets/<replace_with_your_version>/deploy/crds/bundle.yaml" --server-side
 ```
 
 Uncomment the relevant line in the next steps to disable the automatic install of CRDs.