Use a permalink, to reduce the risk of future staleness

Author: mdickinson

Modules/mathintegermodule.c

@@ -182,8 +182,7 @@ iteration to the next. A sketch of the proof of this is given below.
 In addition to the proof sketch, a formal, computer-verified proof
 of correctness (using Lean) of the algorithm can be found here:
 
-    https://github.com/mdickinson/snippets/tree/main/proofs/isqrt
-
+    https://github.com/mdickinson/snippets/tree/41ce2d256fef06fb32f24fe7014cfa95173ac5e0/proofs/isqrt
 
 Here's Python code equivalent to the C implementation below: