Update cert-manager to v0.7 in GKE docs

docs/gitbook/install/flagger-install-on-google-cloud.md

@@ -186,7 +186,7 @@ Install cert-manager's CRDs:
 ```bash
 CERT_REPO=https://raw.githubusercontent.com/jetstack/cert-manager
 
-kubectl apply -f ${CERT_REPO}/release-0.6/deploy/manifests/00-crds.yaml
+kubectl apply -f ${CERT_REPO}/release-0.7/deploy/manifests/00-crds.yaml
 ```
 
 Create the cert-manager namespace and disable resource validation:
@@ -200,10 +200,12 @@ kubectl label namespace cert-manager certmanager.k8s.io/disable-validation=true
 Install cert-manager with Helm:
 
 ```bash
-helm repo update && helm upgrade -i cert-manager \
+helm repo add jetstack https://charts.jetstack.io && \
+helm repo update && \
+helm upgrade -i cert-manager \
 --namespace cert-manager \
---version v0.6.0 \
-stable/cert-manager
+--version v0.7.0 \
+jetstack/cert-manager
 ```
 
 ### Istio Gateway TLS setup