Enhance function def and add docstring

nbcourse/nbcourse.py

@@ -319,7 +319,8 @@ class NbCourse:
                                          self.conf['slug_title'])})],
         }
 
-    def build(self, args=None):
+    def build(self, args: list = None):
+        """Build course using Doit"""
         if args is None:
             args = []
         doit_config = {}  # TODO: implement verbosity option