Fix readme usage instructions

README.md

@@ -13,15 +13,15 @@ This demo only runs with addition puzzles.
 # Usage
 To run this demo, execute:
 ```
-python cryptarithm.py path/to/your/file.txt
+python cryptarithm.py --filename path/to/your/file.txt
 ```
 
 For simplicity and to establish convention, ensure all letters are capitalized,
 that the puzzle be written on the first line of the file, and that no more than 10 unique letters are used in total.
 
 For example:
 ```
-python cryptarithm.py puzzle_files/example1.txt
+python cryptarithm.py --filename puzzle_files/example1.txt
 ```
 
 Could produce: