fix: Broken link in docs (#76)

#### Relevant issue or PR
N/A

#### Description of changes
See title.

#### Testing done
CI passes.

Author: nmheim

examples/fem-shapeopt/demo.ipynb

@@ -21,7 +21,7 @@
     "\n",
     "## Introduction\n",
     "\n",
-    "In this notebook, we explore the optimization of a parametric structure made of a linear elastic material. The structure is parametrized by N bars, each of which has M piecewise linear segments. We seek the ideal configuration of the $y$-coordinates of the vertices that connect those bar segments. This notebook is based on the [2D topology optimization example](https://github.com/deepmodeling/jax-fem/tree/main/demos/topology_optimization) from `jax-fem`, but we solve the problem using a parametric approach instead.\n",
+    "In this notebook, we explore the optimization of a parametric structure made of a linear elastic material. The structure is parametrized by N bars, each of which has M piecewise linear segments. We seek the ideal configuration of the $y$-coordinates of the vertices that connect those bar segments. This notebook is based on the [2D topology optimization example](https://github.com/deepmodeling/jax-fem/blob/main/docs/source/learn/topology_optimization.ipynb) from `jax-fem`, but we solve the problem using a parametric approach instead.\n",
     "\n",
     "**That is, we use end-to-end automatic differentiation (AD) through several components to optimize the design variables directly with respect to (simulated) performance of the design.**\n",
     "\n",