update doc-string of Agent class (#1187)

make it clear that the word slot refers to instance attribute, so it won't be confused with __slots__ magic.

Author: Ewindar

agents.py

@@ -67,17 +67,17 @@ def display(self, canvas, x, y, width, height):
 
 
 class Agent(Thing):
-    """An Agent is a subclass of Thing with one required slot,
-    .program, which should hold a function that takes one argument, the
-    percept, and returns an action. (What counts as a percept or action
+    """An Agent is a subclass of Thing with one required instance attribute 
+    (aka slot), .program, which should hold a function that takes one argument,
+    the percept, and returns an action. (What counts as a percept or action 
     will depend on the specific environment in which the agent exists.)
-    Note that 'program' is a slot, not a method. If it were a method,
-    then the program could 'cheat' and look at aspects of the agent.
-    It's not supposed to do that: the program can only look at the
-    percepts. An agent program that needs a model of the world (and of
-    the agent itself) will have to build and maintain its own model.
-    There is an optional slot, .performance, which is a number giving
-    the performance measure of the agent in its environment."""
+    Note that 'program' is a slot, not a method. If it were a method, then the
+    program could 'cheat' and look at aspects of the agent. It's not supposed
+    to do that: the program can only look at the percepts. An agent program
+    that needs a model of the world (and of the agent itself) will have to
+    build and maintain its own model. There is an optional slot, .performance,
+    which is a number giving the performance measure of the agent in its
+    environment."""
 
     def __init__(self, program=None):
         self.alive = True