Fix typo in CorryvreckanWriter

Separate two detectors in geo file with line break also if a role is assigned.

Before:

...
[fancyDetector]
some_parameter = 0 0 0
role = "reference"
[lameDetector]
...

After:

...
[fancyDetector]
some_parameter = 0 0 0
role = "reference"

[lameDetector]
...