guide user to run "helm repo update" after "helm repo add pingcap https://charts.pingcap.org/"

File: /release-1.4/en/get-started.md

➜  _dev git:(master) ✗ helm install --namespace tidb-admin tidb-operator pingcap/tidb-operator --version v1.4.2
Error: INSTALLATION FAILED: failed to download "pingcap/tidb-operator" at version "v1.4.2"
➜  _dev git:(master) ✗ helm repo update
Hang tight while we grab the latest from your chart repositories...
...Successfully got an update from the "crossplane-stable" chart repository
...Successfully got an update from the "deliveryhero" chart repository
...Successfully got an update from the "pingcap" chart repository
Update Complete. ⎈Happy Helming!⎈
➜  _dev git:(master) ✗ helm install --namespace tidb-admin tidb-operator pingcap/tidb-operator --version v1.4.2
NAME: tidb-operator
LAST DEPLOYED: Tue Feb 21 16:47:15 2023
NAMESPACE: tidb-admin
STATUS: deployed
REVISION: 1
TEST SUITE: None
NOTES:
Make sure tidb-operator components are running:

    kubectl get pods --namespace tidb-admin -l app.kubernetes.io/instance=tidb-operator
➜  _dev git:(master) ✗

Update release-1.4/en/get-started.md, the guide linked in the issue. Start by reviewing the Helm installation commands around adding the pingcap repository, then include the helm repo update step before installing tidb-operator. Done means the documented sequence reflects the working command order shown in the issue.