Update rng usage in eigen_I.md (#735)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Chihiro2000GitHub

lectures/eigen_I.md

@@ -1005,7 +1005,8 @@ A = np.array([[1, 0, 3],
 num_iters = 20
 
 # Define a random starting vector b
-b = np.random.rand(A.shape[1])
+rng = np.random.default_rng()
+b = rng.random(A.shape[1])
 
 # Get the leading eigenvector of matrix A
 eigenvector = np.linalg.eig(A)[1][:, 0]