[5.0.x] Fixed typo in docs/internals/contributing/writing-code/coding-style.txt.

Backport of aaffbabd58c8f3c9bf82cd4eb98b8c5cb9e7aa6a from main

docs/internals/contributing/writing-code/coding-style.txt

@@ -310,8 +310,8 @@ Model style
 
     class MyModel(models.Model):
         class Direction(models.TextChoices):
-            UP = U, "Up"
-            DOWN = D, "Down"
+            UP = "U", "Up"
+            DOWN = "D", "Down"
 
 Use of ``django.conf.settings``
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