Fix typo in filename (#54)

Author: fhinkel

.github/steps/3-customize-codespace.md

@@ -10,7 +10,7 @@ _Nice work! :tada: You created a codespace with a custom image!_
 
 You can customize your codespace by adding VS code extensions, adding features, setting host requirements, and much more.
 
-Let's customize some settings in the `.devcontainer.json` file!
+Let's customize some settings in the `devcontainer.json` file!
 
 ### :keyboard: Activity: Add customizations to the `devcontainer` file