docs(0005): typo fix in the challenge docs (rust-lang#178)

This PR, just fixes a typo for a function name in the challenge docs.

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doc/src/challenges/0005-linked-list.md

@@ -23,7 +23,7 @@ The memory safety of the following public functions that iterating over the inte
 
 | Function | Location |
 |---------|---------|
-|clearn | alloc::collections::linked_list |
+|clear| alloc::collections::linked_list |
 |contains| alloc::collections::linked_list |
 |split_off| alloc::collections::linked_list |
 |remove| alloc::collections::linked_list |