remove mention of sequential collections from append file

exercises/practice/sublist/.docs/instructions.append.md

@@ -8,4 +8,4 @@ This appendix provides additional clarification or modifies the instructions as
 ~~~~
 
 For this exercise in the Clojure track, you may **assume the input is a vector**, as indicated by the tests.
-However, this is not a strict requirement; you are free to treat the input as a sequential collection, without assuming it is specifically a vector.
+However, this is not a strict requirement.