[docs] Fix openvpn backend: remote.port should be int

[asmodehn]

Checklist

• [X] I have read the OpenWISP Contributing Guidelines.
• [X] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [X] I have updated the documentation.

Reference to Existing Issue

The schema says : https://github.com/openwisp/netjsonconfig/blob/master/netjsonconfig/backends/openvpn/schema.py#L615

Description of Changes

Fix openvpn backend documentation for remote.port