DOC: Fix v4 link in header

Author: VeckoTheGecko

docs/index.rst

@@ -17,7 +17,7 @@ If you need more help with Parcels, try the `Discussions page on GitHub <https:/
    :hidden:
 
    Home <self>
-   v4 <https://docs.oceanparcels.org/en/v4-dev/v4>
+   v4 <https://docs.oceanparcels.org/en/main/v4>
    Installation <installation>
    Tutorials & Documentation <documentation/index>
    API reference <reference>