Fix #247 - explicitly declare position_proportional_gain parameter (backport #248)

[mergify]

With this patch, you can change position gain with this config file lines:

gz_ros_control:
  ros__parameters:
    position_proportional_gain: 0.5
```<hr>This is an automatic backport of pull request #248 done by [Mergify](https://mergify.com).

[destogl]

@ahcorde I am having issues with this fix on humble crashing with error:

parameter 'position_proportional_gain' has already been declared

After reverting this commit, the simulation starts as expected.