update scalar

Author: HumphreyYang

lectures/muth_kalman.md

@@ -176,7 +176,7 @@ kmuth = Kalman(ss, x_hat_0, Σ_0)
 # representation
 S1, K1 = kmuth.stationary_values()
 
-# Extract scalars from the nested array
+# Extract scalars from nested arrays
 S1, K1 = S1.item(), K1.item()
 
 # Form innovation representation state-space