Fixes README markdown for copying code instructions from clipboard

Currently, when trying to follow instructions from README is very cumbersome, as everything from the codeblock gets copied when trying to do this from markdown:

 ```text
  $ vault secrets enable -path=venafi-pki -plugin-name=venafi-pki-backend plugin
  Success! Enabled the pki-backend-venafi secrets engine at: venafi-pki/


This will copy in clipboard everything:

$ vault secrets enable -path=venafi-pki -plugin-name=venafi-pki-backend plugin Success! Enabled the pki-backend-venafi secrets engine at: venafi-pki/


When just rather want to copy:

vault secrets enable -path=venafi-pki -plugin-name=venafi-pki-backend plugin


As of today, Github doesn't offer a fine solution like suggested here:
https://github.com/orgs/community/discussions/35615#discussioncomment-3834745

to do something like:

$ echo Foo
Foo


which would solve our case.

Checking the rest of the issue here:
https://github.com/orgs/community/discussions/35615
we can verify this is is not yet supported by Github.

Ideally we would want to copy  onlythe command and not the dollar sign or the output, like, for example, done in 
HashiCorp guides (taking for example, guide for starting a minimal dev server):
https://developer.hashicorp.com/vault/tutorials/getting-started/getting-started-dev-server

Until then, this is the best and quickest workaround I propose to solve this problem in the very short-term until Github provides a better way to achieve this natively.

You can verify how my changes would look like once this PR is merged, here:

https://github.com/Venafi/vault-pki-backend-venafi/tree/readme-instructions-fix

Venafi / vault-pki-backend-venafi

Fixes README markdown for copying code instructions from clipboard #152