Update README.md link to f4pga readthedocs

Signed-off-by: Bastian Löher <me@l-dot.de>

README.md

diff --git a/README.md b/README.md
index 5d44fc1..7ac8abb 100644
--- a/README.md
+++ b/README.md
@@ -244,7 +244,7 @@
 
 ## Adding a new part to an existing family
 
-We have written a [detailed guide](https://symbiflow.readthedocs.io/projects/prjxray/en/latest/db_dev_process/newpart.html) that walks through the process of adding a new part to an existing family.
+We have written a [detailed guide](https://f4pga.readthedocs.io/projects/prjxray/en/latest/db_dev_process/newpart.html) that walks through the process of adding a new part to an existing family.
 
 ## TODO List