Merge pull request #144 from wilsonrljr/fix/readme_book_image

fix cover book link in readme.

README.md

@@ -50,7 +50,7 @@ Check our [documentation](https://sysidentpy.org)!
 For an in depth documentation, check our companion book:
 
 <a href="https://sysidentpy.org/book/0%20-%20Preface/">
-  <img src="./docs/book/assets/Nonlinear_System_identification.png" alt="Nonlinear System Identification" style="width: 200px; height: auto;" />
+  <img src="https://github.com/wilsonrljr/sysidentpy-data/blob/4085901293ba5ed5674bb2911ef4d1fa20f3438d/book/assets/Nonlinear_System_identification.png?raw=true" alt="Nonlinear System Identification" style="width: 200px; height: auto;" />
 </a>
 
 
@@ -172,7 +172,7 @@ Regressors     Parameters        ERR
 plot_results(y=y_valid, yhat=yhat, n=1000)
 ee = compute_residues_autocorrelation(y_valid, yhat)
 plot_residues_correlation(data=ee, title="Residues", ylabel="$e^2$")
-x1e = compute_cross_correlation(y_valid, yhat, x2_val)
+x1e = compute_cross_correlation(y_valid, yhat, x_valid)
 plot_residues_correlation(data=x1e, title="Residues", ylabel="$x_1e$")
 ```
 ![polynomial](./examples/figures/polynomial_narmax.png)