Update tutorial serialization page (#7945)

Author: juliangeissler

docs/tutorial/1-serialization.md

@@ -15,7 +15,7 @@ Before we do anything else we'll create a new virtual environment, using [venv].
 
 ```bash
 python3 -m venv env
-source env/bin/activate
+source env/bin/activate   # On Windows use `env\Scripts\activate`
 ```
 
 Now that we're inside a virtual environment, we can install our package requirements.
@@ -217,6 +217,10 @@ Let's look at refactoring our serializer using the `ModelSerializer` class.
 Open the file `snippets/serializers.py` again, and replace the `SnippetSerializer` class with the following.
 
 ```python
+from rest_framework import serializers
+from snippets.models import Snippet
+
+
 class SnippetSerializer(serializers.ModelSerializer):
     class Meta:
         model = Snippet
@@ -362,9 +366,9 @@ Quit the server with CONTROL-C.
 
 In another terminal window, we can test the server.
 
-We can test our API using [curl][curl] or [httpie][httpie]. Httpie is a user friendly http client that's written in Python. Let's install that.
+We can test our API using [curl][curl] or [HTTPie][HTTPie]. HTTPie is a user-friendly http client that's written in Python. Let's install that.
 
-You can install httpie using pip:
+You can install HTTPie using pip:
 
 ```bash
 pip install httpie
@@ -436,5 +440,5 @@ We'll see how we can start to improve things in [part 2 of the tutorial][tut-2].
 [repo]: https://github.com/encode/rest-framework-tutorial
 [venv]: https://docs.python.org/3/library/venv.html
 [tut-2]: 2-requests-and-responses.md
-[httpie]: https://github.com/httpie/httpie#installation
+[HTTPie]: https://github.com/httpie/httpie#installation
 [curl]: https://curl.haxx.se/