Fix missing return value for ObservableArray "set the length"

Closes #1050.

Author: domenic

index.bs

@@ -14052,6 +14052,7 @@ those of normal <code>Array</code> instances:
             |handler|.\[[BackingList]][|indexToDelete|] and |indexToDelete|.
         1.  [=list/Remove=] the last item from |handler|.\[[BackingList]].
         1.  Set |indexToDelete| to |indexToDelete| &minus; 1.
+    1.  Return <emu-val>true</emu-val>.
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