docs: Typo fix in multi-window docs

Referred to the "android:resizableActivity" property; should
have been "android:resizeableactivity". English prose, however,
consistently uses the (preferred) spelling "resizable".

See first comment for stage location.

bug: 30566808
Change-Id: I2570ff9269b4d2dc8b11f1c98817e50726be78fc