standardize module docstring

pina/equation/__init__.py

@@ -1,6 +1,4 @@
-"""
-Module to define equations and systems of equations.
-"""
+"""Module to define equations and systems of equations."""
 
 __all__ = [
     "SystemEquation",

pina/equation/equation_interface.py

@@ -1,4 +1,4 @@
-"""Module for the Equation Interface"""
+"""Module for the Equation Interface."""
 
 from abc import ABCMeta, abstractmethod