Uniform text with the rest of the course

In all other parts of the course the term used for this kind of activity is "exercise", while in this part the term "exercise" and the term "task" are used interchangeably.

Author: micpob

src/content/13/en/part13a.md

@@ -594,17 +594,17 @@ app.post('/api/notes', async (req, res) => {
 
 <div class="tasks">
 
-### Tasks 13.1.-13.3.
+### Exercises 13.1.-13.3.
 
 In the tasks of this section, we will build a blog application backend similar to the tasks in [section 4](/en/part4), which should be compatible with the frontend in [section 5](/en/part5) except for error handling. We will also add various features to the backend that the frontend in section 5 will not know how to use.
 
-#### Task 13.1.
+#### Exercise 13.1.
 
 Create a GitHub repository for the application and create a new Fly.io or Heroku application for it, as well as a Postgres database. As mentioned [here](/en/part13/using_relational_databases_with_sequelize#application-database) you might set up your database also somewhere else, and in that case the Fly.io of Heroku app is not needed.
 
 Make sure you are able to establish a connection to the application database.
 
-#### Task 13.2.
+#### Exercise 13.2.
 
 On the command-line, create a <i>blogs</i> table for the application with the following columns:
 - id (unique, incrementing id)
@@ -846,9 +846,9 @@ The print looks like the following:
 
 <div class="tasks">
 
-### Task 13.4.
+### Exercise 13.4.
 
-#### Task 13.4.
+#### Exercise 13.4.
 
 Transform your application into a web application that supports the following operations