Add materialize.md to documentation list

Issue Reactive-Extensions#849 (Reactive-Extensions#849) was closed, but the created documentation for it is currently not available via the observable documentation.

Author: jackmeagher

doc/api/core/observable.md

@@ -113,6 +113,7 @@ The Observer and Observable interfaces provide a generalized mechanism for push-
 - [`let`](operators/let.md)
 - [`manySelect`](operators/manyselect.md)
 - [`map`](operators/select.md)
+- [`materialize`](operators/materialize.md)
 - [`max`](operators/max.md)
 - [`maxBy`](operators/maxby.md)
 - [`merge`](operators/mergeproto.md)