Update rng usage in os_egm.md (#996)

Claude-Session: https://claude.ai/code/session_01Rs2tDEcDXLnfMnM88v8e1D

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Chihiro2000GitHub

lectures/os_egm.md

@@ -210,8 +210,8 @@ def create_model(
     s_grid = np.linspace(1e-4, grid_max, grid_size)
 
     # Store shocks (with a seed, so results are reproducible)
-    np.random.seed(seed)
-    shocks = np.exp(μ + ν * np.random.randn(shock_size))
+    rng = np.random.default_rng(seed)
+    shocks = np.exp(μ + ν * rng.standard_normal(shock_size))
 
     return Model(
         u, f, β, μ, ν, s_grid, shocks, α, u_prime, f_prime, u_prime_inv