Documentation: Correct certificate file format for az eventgrid namespace ca-certificate create

**Related command**

`az eventgrid namespace ca-certificate create`

**Is your feature request related to a problem? Please describe.**

The command only accepts the PEM certificate file in single line form, even though this is not the most spec compliant form (that's fine), but no guidance is given on the file format (not fine).

**Describe the solution you'd like**

Please can an example file be shown on the documentation pages (API docs, cli docs), so that people have an easy time getting the format right.

Alternatively, perhaps note the (bash) transform required to get it into the right shape.

`cat ./intermediate-ca-input.pem | tr -d '\n' > ./intermediate-ca-output.pem`

**Describe alternatives you've considered**
The REST API response could be more informative about what is wrong with a specific upload attempt.

**Additional context**
This cost me a couple hours yesterday and it's an obstacle to automation, which means it is an obstacle to good security practice. This genuinely could be made more usable for thousands of people with a couple lines of commentary about correct format.