docs: correct link to multiprocessing docs (#1023)

* docs: correct link to multiprocessing docs

Signed-off-by: Jason Mobarak <git@jason.mobarak.name>

* Update docs/content/exporting/http/fastapi-gunicorn.md

Co-authored-by: Chris Marchbanks <csmarchbanks@gmail.com>
Signed-off-by: Jason Mobarak <git@jason.mobarak.name>

---------

Signed-off-by: Jason Mobarak <git@jason.mobarak.name>
Co-authored-by: Chris Marchbanks <csmarchbanks@gmail.com>

docs/content/exporting/http/fastapi-gunicorn.md

@@ -19,7 +19,7 @@ metrics_app = make_asgi_app()
 app.mount("/metrics", metrics_app)
 ```
 
-For Multiprocessing support, use this modified code snippet. Full multiprocessing instructions are provided [here](https://github.com/prometheus/client_python#multiprocess-mode-eg-gunicorn).
+For Multiprocessing support, use this modified code snippet. Full multiprocessing instructions are provided [here]({{< ref "/multiprocess" >}}).
 
 ```python
 from fastapi import FastAPI
@@ -47,4 +47,4 @@ pip install gunicorn
 gunicorn -b 127.0.0.1:8000 myapp:app -k uvicorn.workers.UvicornWorker
 ```
 
-Visit http://localhost:8000/metrics to see the metrics
+Visit http://localhost:8000/metrics to see the metrics