In the version of the article initially published, the scale bar in Fig. 3d was labelled "Proton density (10 -5 -3 )" and read (from top to bottom) "1.0, 0.5, 0" but should have been labelled "Proton density (10 -1 -3 )" and have read (from top to bottom) "1.7, 0.7, 0".In the Fig.