Added functionality to handle line breaks in dt tag

[Prakharkarsh1]

Changes has been done in ggroff.py file in order to fix the issue #2129. Inserted a condition which checks if .br is present in string format (spec) with \n and replacing the format (spec) with simple space ("\n.br\n" to ".br ")

[wenzeslaus]

This removed the warning, but it results in text .br in the man page:

       polygon  .br    xpos ypos  .br    xpos ypos  .br   ...
           The coordinates appearing beneath the word polygon, one pair per line, circumscribe a polygon that is to be filled with the current color.

[wenzeslaus]

Man page:

grass --tmp-location XY --exec man d.graph