Update astro.rst -- use latest Astro Runtime (astronomer#988)

## Description

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## Related Issue(s)

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## Breaking Change?

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## Checklist

- [ ] I have made corresponding changes to the documentation (if
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- [ ] I have added tests that prove my fix is effective or that my
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docs/getting_started/astro.rst

@@ -25,7 +25,7 @@ Create a virtual environment in your ``Dockerfile`` using the sample below. Be s
 
 .. code-block:: docker
 
-    FROM quay.io/astronomer/astro-runtime:10.0.0
+    FROM quay.io/astronomer/astro-runtime:11.3.0
 
     # install dbt into a virtual environment
     RUN python -m venv dbt_venv && source dbt_venv/bin/activate && \