docs: fix incorrect spectral radius computation in README example

[sunnyc0206]

This PR corrects the stability diagnostic shown in the README example for the recurrent block.

The current example uses A.max().item() to represent the spectral radius of matrix A, which is mathematically incorrect. Stability in the recurrent formulation is governed by the spectral radius ρ(A), defined as the maximum absolute eigenvalue of A, not its maximum element.

This update replaces the incorrect computation with the correct formulation:

rho = torch.linalg.eigvals(A).abs().max().item()

and updates the accompanying print statement accordingly.

Rationale:
The README specifies that stability requires ρ(A)<1. The previous implementation did not compute this quantity, which could lead to incorrect interpretation of model stability during experimentation or debugging.

Impact:

• Documentation/example accuracy improved
• Aligns README with stated theoretical stability condition
• No changes to model implementation or runtime behavior
• No breaking changes

#51