Add missing slugs to 4 lectures

Author: jslvtr

docs/docs/05_flask_smorest/03_improvements_on_first_rest_api/README.md

@@ -1,8 +1,7 @@
 ---
 title: "Improvements to our first REST API"
 description: "Add new error handling and code improvements to the REST API before adding any new endpoints."
-
-slugs: improvements-to-our-first-rest-api
+slug: improvements-to-our-first-rest-api
 ---
 
 # Improvements to our first REST API

docs/docs/05_flask_smorest/04_new_endpoints_for_api/README.md

@@ -1,8 +1,7 @@
 ---
 title: "New endpoints for our REST API"
 description: "Let's add a few routes to our first REST API, so it better matches what a production REST API would look like."
-
-slugs: new-endpoints-for-our-rest-api
+slug: new-endpoints-for-our-rest-api
 ---
 
 # New endpoints for our REST API

docs/docs/05_flask_smorest/05_reload_api_docker_container/README.md

@@ -1,8 +1,7 @@
 ---
 title: "Reloading API code in Docker container"
 description: "Learn how to get your code instantly synced up to the Docker container, so that every time you make a code change it restarts the app in the container and uses the latest code."
-
-slugs: reloading-api-code-in-docker-container
+slug: reloading-api-code-in-docker-container
 ---
 
 # Reloading API code in Docker container

docs/docs/08_flask_jwt_extended/04_flask_jwt_extended_setup/README.md

@@ -1,6 +1,7 @@
 ---
 title: Flask-JWT-Extended setup
 description: Install and set up the Flask-JWT-Extended extension with our REST API.
+slug: flask-jwt-extended-setup
 ---
 
 # Flask-JWT-Extended setup