fix: routing_rules refresh

[Edu-DevOps]

This PR aims to fix #397.

I have tested:

• creation of new repo with a routing rule specified
• applying after a manual modification of the routing rule using the UI
• tested with multiple kind of repos (proxy all of them since routing rules are only for proxy repos)

This is the output of an apply when the routing rule is manually modified:

nexus_repository_r_proxy.cran_mirror: Refreshing state... [id=cran-mirror]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # nexus_repository_r_proxy.cran_mirror will be updated in-place
  ~ resource "nexus_repository_r_proxy" "cran_mirror" {
        id           = "cran-mirror"
        name         = "cran-mirror"
      + routing_rule = "allow_cran"
        # (1 unchanged attribute hidden)

        # (4 unchanged blocks hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

nexus_repository_r_proxy.cran_mirror: Modifying... [id=cran-mirror]
nexus_repository_r_proxy.cran_mirror: Modifications complete after 0s [id=cran-mirror]

Apply complete! Resources: 0 added, 1 changed, 0 destroyed.