doc: Fix package name in external renderer example (#11170)

The py3-zmq package does not exist in alpine linux 3.11, used in both master and 1.11.4 . The py3-pyzmq package exists however:
https://pkgs.alpinelinux.org/packages?name=py3-pyzmq&branch=v3.11

Author: johanvdw

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 python-dev py-pip python3-dev py3-pip py3-zmq
+RUN apk --no-cache add asciidoctor freetype freetype-dev gcc g++ libpng python-dev py-pip python3-dev py3-pip py3-pyzmq
 # install any other package you need for your external renderers
 
 RUN pip3 install --upgrade pip