Fix code block in README.md (encode#8408)

Hi there,

The code block below show imply `Python` as it lives in `settings.py`

```
INSTALLED_APPS = [
    ...
    'rest_framework',
]
```

This pull request essentially fixes that.

Author: baseplate-admin

README.md

@@ -67,11 +67,12 @@ Install using `pip`...
     pip install djangorestframework
 
 Add `'rest_framework'` to your `INSTALLED_APPS` setting.
-
-    INSTALLED_APPS = [
-        ...
-        'rest_framework',
-    ]
+```python
+INSTALLED_APPS = [
+    ...
+    'rest_framework',
+]
+```
 
 # Example