Modify rule S6984: Update einsum link (#4565)

rules/S6984/python/rule.adoc

@@ -4,7 +4,7 @@ This rule raises an issue when an incorrect pattern is provided to an `einops` o
 == Why is this an issue?
 
 The `einops` library provides a powerful and flexible way to manipulate tensors using the Einstein summation convention.
-The `einops` uses a different convention than the https://rockt.github.io/2018/04/30/einsum[traditional] one.
+The `einops` uses a different convention than the https://ejenner.com/post/einsum/[traditional] one.
 In particular, the axis names can be more than one letter long and are separated by spaces.