input: Write a python function to generate a random matrix with the specified number of rows and columns using numpy.
  assert random_matrix(2,2).shape == (2,2)
  assert random_matrix(3,4).shape == (3,4)
  assert random_matrix(5,1).shape == (5,1)
output: import numpy as np
  
  def random_matrix(rows, cols):
      return np.random.rand(rows, cols)