input: Write a python function to plot a histogram for an array using matplotlib.
  assert plot_histogram([1, 2, 3, 4, 5, 6]) is None
  assert plot_histogram([10, 20, 30, 40, 50]) is None
  assert plot_histogram([1, 1, 2, 2, 3, 3, 4, 4]) is None
output: def plot_histogram(arr):
      import matplotlib.pyplot as plt
      plt.hist(arr, bins='auto', edgecolor='black')
      plt.title('Histogram')
      plt.xlabel('Value')
      plt.ylabel('Frequency')
      plt.show()