fix binder links

examples/toolkit_showcase/README.md

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 ## Toolkit Showcase: Protein-ligand simulation with an OpenFF force field
 
-[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/openforcefield/openff-toolkit/stable?filepath=examples2Ftoolkit_showcase2Ftoolkit_showcase.ipynb)
+[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/openforcefield/openff-toolkit/stable?filepath=examples%2Ftoolkit_showcase%2Ftoolkit_showcase.ipynb)
 
 This introductory example will demonstrate how to prepare a system for Molecular Dynamics that combines solvent, a ligand using the Sage open force field, and a protein using a standard Amber force field. We'll use a number of tools from the Python molecular science ecosystem to take a pre-docked benchmark example from scratch all the way through to simulation and visualization, all without leaving the notebook.