Update external-renderers.en-us.md (#13165)

Alpine 3.12 (the version used by current master) no longer provides the python-dev package:
https://wiki.alpinelinux.org/wiki/Release_Notes_for_Alpine_3.12.0#python2_no_longer_provides_python_and_python-devel

As the next steps explicitely use python3 it is not needed anyway.

docs/content/doc/advanced/external-renderers.en-us.md

@@ -36,7 +36,7 @@ FROM gitea/gitea:{{< version >}}
 COPY custom/app.ini /data/gitea/conf/app.ini
 [...]
 
-RUN apk --no-cache add asciidoctor freetype freetype-dev gcc g++ libpng libffi-dev python-dev py-pip python3-dev py3-pip py3-pyzmq
+RUN apk --no-cache add asciidoctor freetype freetype-dev gcc g++ libpng libffi-dev py-pip python3-dev py3-pip py3-pyzmq
 # install any other package you need for your external renderers
 
 RUN pip3 install --upgrade pip