Add examples for opener argument of open (#13424).

Patch by Guillaume Pratte.
diff --git a/Misc/ACKS b/Misc/ACKS
index ee1ce67..81ed27c 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -929,6 +929,7 @@
 Iustin Pop
 Claudiu Popa
 John Popplewell
+Guillaume Pratte
 Amrit Prem
 Paul Prescod
 Donovan Preston