Update formatting in instructions.append.md for Difference of Squares (…

…#1235)

exercises/practice/difference-of-squares/.docs/instructions.append.md

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 # Hints
 
 For this exercise the following F# features come in handy:
-https://fsharp.github.io/fsharp-core-docs/reference/fsharp-core-operators.html#(%20..%20)) allows you to succinctly create a range of values.
+- [(..) start finish](https://fsharp.github.io/fsharp-core-docs/reference/fsharp-core-operators.html#(..)) allows you to succinctly create a range of values.
 - [List.sumBy](https://fsharp.github.io/fsharp-core-docs/reference/fsharp-collections-listmodule.html#sumBy) is a condensed format to apply a function to a list and then sum the results.