Update acrobot docstrings (#2362)

Removed time from the signature of derivs in the documentation. Also fixed a typo and removed an example that's no longer valid

gym/envs/classic_control/acrobot.py

@@ -273,31 +273,26 @@ def rk4(derivs, y0, t):
  :func:`scipy.integrate`.
 
  Args:
- derivs: the derivative of the system and has the signature ``dy = derivs(yi, ti)``
+ derivs: the derivative of the system and has the signature ``dy = derivs(yi)``
  y0: initial state vector
  t: sample times
  args: additional arguments passed to the derivative function
  kwargs: additional keyword arguments passed to the derivative function
 
  Example 1 ::
  ## 2D system
- def derivs6(x,t):
+ def derivs(x):
  d1 = x[0] + 2*x[1]
  d2 = -3*x[0] + 4*x[1]
  return (d1, d2)
  dt = 0.0005
  t = arange(0.0, 2.0, dt)
  y0 = (1,2)
  yout = rk4(derivs6, y0, t)
- Example 2::
- ## 1D system
- alpha = 2
- def derivs(x,t):
- return -alpha*x + exp(-t)
- y0 = 1
- yout = rk4(derivs, y0, t)
+
  If you have access to scipy, you should probably be using the
- scipy.integrate tools rather than this function.
+ scipy.integrate tools rather than this function. 
+ This would then require re-adding the time variable to the signature of derivs.
 
  Returns:
  yout: Runge-Kutta approximation of the ODE