Update usage.rst

Fix formatting of usage.rst

docs/usage.rst

@@ -195,7 +195,7 @@ More options for customizing the graphs can be found by running ``lpath plot --h
 Example Reassign file
 ---------------------
 
-The following is a reassign function if you decides to reclassify your states.
+The following is a reassign function if you decides to reclassify your states::
 
     def reassign_custom(data, pathways, dictionary, assign_file=None):
         """
@@ -254,4 +254,4 @@ The following is a reassign function if you decides to reclassify your states.
         # Generating a dictionary mapping each state
         dictionary = {0: 'A', 1: 'B', 2: '!'}
 
-        return dictionary
+        return dictionary