[scalar_dynam] Add an example to local stability

[longye-tian]

Dear John @jstac ,

According to #426, we need an example for local stability that is not globally stable.

I propose to use example in DP book 1, as follows

Consider the self-map $g$ on $\mathbb{R}$ defined by $g(x)=x^2$. The fixed point $1$ is not stable.

For example, $g^t(x)\to\infty$ for any $x>1$.

However, $0$ is locally stable, because $-1<x<1$ implies that $g^t(x)\to 0$ as $t\to \infty$.

Best ❤️
Lonyge

[longye-tian]

This issue is now closed by #523.