Update changelog with bug 296

Author: igaztanaga

doc/container.qbk

@@ -1431,6 +1431,7 @@ use [*Boost.Container]? There are several reasons for that:
    * [@https://github.com/boostorg/container/issues/261 GitHub #261: ['"End iterators are not dereferencable"]].
    * [@https://github.com/boostorg/container/issues/288 GitHub #288: ['"Compile error when using flat_map::extract_sequence with small_vector"]].
    * [@https://github.com/boostorg/container/issues/295 GitHub #295: ['"Bug in small_vector::swap"]].
+   * [@https://github.com/boostorg/container/issues/296 GitHub #296: ['"flat_{set,map} does illegal things when confronted with duplicate values in initialization"]].
 
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