Fix typo in docs: delete extra backtick (#868)

Author: jasongrout

docs/user_guide/features/extension.md

@@ -5,7 +5,7 @@
 JupyterGIS comes with a JupyterLab extension that embeds QGIS features directly into the JupyterLab UI.
 This extension allows you to:
 
-- Open `.qgz` and `` .jGIS` `` files in JupyterLab
+- Open `.qgz` and `.jGIS` files in JupyterLab
 - Create new `.jGIS` files
 - Edit those files by adding new vector or raster layers, modifying geometries, and performing spatial operations.