Fix redirect url in oidc.md (#61)

Fixed redirect url with new one from more recent release (don't know which one, just found out that it didn't work after an update).

Author: Allexio

docs/en/install_setup/oidc.md

@@ -55,7 +55,7 @@ Ensure that regex is enabled if your provider requires it (e.g., Authentik).
 
 **For OIDC providers that do not support wildcards: (e.g., Authelia)**
 
-- `https://your-gramps-backend.com/api/oidc/callback/?provider=custom`
+- `https://your-gramps-backend.com/api/oidc/callback/custom`
 
 ## Role Mapping