Make sure docstring lines are joined with a space

[lohedges]

This PR fixes the formatting of the help message printed by the CLI by joining the docstring lines for Config parameters with a space character. Previously, the lines were just appended so there was no space between.

Before:

  --perturbable-constraint {none,auto,h_bonds,bonds,h_bonds_h_angles,bonds_h_angles}
                        Constraint type to use for perturbable molecules. If None, thenthis will be set according to what is chosen for
                        thenon-perturbable constraint. (default: None)

After:

  --perturbable-constraint {none,auto,h_bonds,bonds,h_bonds_h_angles,bonds_h_angles}
                        Constraint type to use for perturbable molecules. If None, then this will be set according to what is chosen for
                        the non-perturbable constraint. (default: None)

[mb2055]

Good catch