doc: update version number for assert function introduction

Author: miguelmarcondesf

doc/api/assert.md

@@ -243,7 +243,7 @@ assertInstance.deepStrictEqual({ a: 1 }, { a: 2 });
 ## `assert(value[, message])`
 
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-added: v0.1.21
+added: v0.5.9
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 * `value` {any} The input that is checked for being truthy.