Fix BasicOperation.playground in Xcode 12.2.

Author: pitfield

Playgrounds/BasicOperation.playground/contents.xcplayground

@@ -1,4 +1,4 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
-<playground version='5.0' target-platform='macos' last-migration='1020'>
+<playground version='5.0' target-platform='macos' buildActiveScheme='true' last-migration='1020'>
     <timeline fileName='timeline.xctimeline'/>
 </playground>