Fixed typo in comments (aimacode#302)

Author: doctorBeast

planning.py

@@ -60,7 +60,7 @@ def __call__(self, kb, args):
         return self.act(kb, args)
 
     def substitute(self, e, args):
-        """Replaces variables in expression with their respective Propostional symbol"""
+        """Replaces variables in expression with their respective Propositional symbol"""
         new_args = list(e.args)
         for num, x in enumerate(e.args):
             for i in range(len(self.args)):