Fix table in 0025-vecdeque.md (model-checking#425)

Fixes a broken Markdown table in the Challenge 25 description.

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Author: btj

doc/src/challenges/0025-vecdeque.md

@@ -38,6 +38,8 @@ Write and prove the contract for the safety of the following unsafe functions:
 
 Prove the absence of undefined behavior for following safe abstractions:
 
+| Function |
+|---------|
 |get|
 |get_mut|
 |swap|