Fix docs about stack group config in templates (Sceptre#963)

A [test case](https://github.com/Sceptre/sceptre/blob/40829c0b6de08816a2fc1073101d2da18d88d259/tests/fixtures/config/account/stack-group/region/security_groups.yaml#L5) suggests that change
alex-harvey-z3q · Apr 2, 2021 · 881c6bd · 881c6bd
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commit 881c6bd
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/docs/_source/docs/stack_config.rst b/docs/_source/docs/stack_config.rst
@@ -223,7 +223,7 @@ when using templating.
 .. code-block:: yaml
 
    parameters:
-     sceptre-project-code: {{ stack_group_config.sceptre-project-code }}
+     sceptre-project-code: {{ stack_group_config.project-code }}
 
 Environment Variables
 ---------------------