Update logic.py fix minor typos (#474)

ltfschoen · norvig · commit 5ecee13fb7a1 · 2017-04-06T10:32:29.000-07:00
diff --git a/logic.py b/logic.py
@@ -13,7 +13,7 @@
 Logical expressions can be created with Expr or expr, imported from utils, TODO
 or with expr, which adds the capability to write a string that uses
 the connectives ==>, <==, <=>, or <=/=>. But be careful: these have the
-opertor precedence of commas; you may need to add parens to make precendence work.
+operator precedence of commas; you may need to add parens to make precedence work.
 See logic.ipynb for examples.
 
 Then we implement various functions for doing logical inference:
