In documentation for Helm chart replace command for Identity.Issuer.CrtExpiry

Nobody has claimed this yet.

## Bug Report

### What is the issue?

https://linkerd.io/2/tasks/install-helm/#helm-install-procedure
and https://linkerd.io/2/tasks/generate-certificates/

```bash
# set expiry date one year from now, in Mac:
exp=$(date -v+8760H +"%Y-%m-%dT%H:%M:%SZ")
# in Linux:
exp=$(date -d '+8760 hour' +"%Y-%m-%dT%H:%M:%SZ")
```

The command does not look perfect, because with each call it generates a new value and does not correspond to the expiration date in CrtPEM as it is required in `values.yaml`:

```yaml
# must match the expiry date in CrtPEM
CrtExpiry:
```

### Possible solution

I suggest to replace it with something like this:

```bash
date -d"$(cat issuer.crt | openssl x509 -noout -enddate | sed 's/notAfter=//')" +"%Y-%m-%dT%H:%M:%SZ"
2020-09-05T09:11:25Z

# which is equal to
cat issuer.crt | openssl x509 -noout -enddate | sed 's/notAfter=//'
Sep 5 09:11:25 2020 GMT

# versus
date -d '+8760 hour' +"%Y-%m-%dT%H:%M:%SZ
2020-09-05T11:03:18Z
```

Not sure about the syntax for OSX.

Open the Helm install procedure and certificate-generation pages linked in the issue, then compare the documented expiry command with the CrtPEM requirement in values.yaml. Verify the proposed certificate-derived command on Linux and determine the corresponding OSX syntax. Done means both pages show a command that derives CrtExpiry from issuer.crt and remains consistent with the certificate expiry.

Written by the indexing model from the issue text.