[python_api.md] Fix variable name in Python doc (#31)

Author: SimsGautam

docs/python_api.md

@@ -14,8 +14,8 @@ Constructing the environment, doing one step and retrieving one observation:
 import deepmind_lab
 
 # Construct and start the environment.
-lab = deepmind_lab.Lab('seekavoid_arena_01', ['RGB_INTERLACED'])
-lab.reset()
+env = deepmind_lab.Lab('seekavoid_arena_01', ['RGB_INTERLACED'])
+env.reset()
 
 # Create all-zeros vector for actions.
 action = np.zeros([7], dtype=np.intc)