Update letters.py

lec2/letters.py

@@ -30,7 +30,7 @@ def file2pairs(filename):
     letter_dict  = { c: { a: 0 for a in letters }
                      for c in letters }
     previous = None
-    with open(filename, encdoing="utf-8") as fp:
+    with open(filename, encoding="utf-8") as fp:
         for c in fp.read():
             if c not in letter_dict:
                 continue