fix typo

doc/index.html

@@ -245,7 +245,7 @@ axes should represent.  This can either be done in terms of the old
 axes (<kbd>x y z -x -y -z</kbd>) or in terms of anatomical labels
 <i>when this information is available</i> (in a nifti image where
 either the sform or qform code is non-zero).  The anatomical labels
-are RL LR AP PA SI IA. Note that the anatomical label version will not
+are RL LR AP PA SI IS. Note that the anatomical label version will not
 allow the left-right convention to be changed.
 
 <p>