DOC: Fix old reference to Image Number
DOC: Fix old reference to Image Number

Before DICOM standard, the Instance Number (0020,0013) was referenced as 'Image Number'. This turns out to confuse users. Fix this.
thewtex/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,richardbeare/ITK,BRAINSia/ITK,Kitware/ITK,richardbeare/ITK,LucasGandel/ITK,hjmjohnson/ITK,BRAINSia/ITK,richardbeare/ITK,malaterre/ITK,richardbeare/ITK,vfonov/ITK,thewtex/ITK,BRAINSia/ITK,vfonov/ITK,malaterre/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,vfonov/ITK,malaterre/ITK,LucasGandel/ITK,hjmjohnson/ITK,BRAINSia/ITK,thewtex/ITK,LucasGandel/ITK,malaterre/ITK,LucasGandel/ITK,Kitware/ITK,thewtex/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,malaterre/ITK,hjmjohnson/ITK,malaterre/ITK,BRAINSia/ITK,malaterre/ITK,malaterre/ITK,thewtex/ITK,Kitware/ITK,richardbeare/ITK,vfonov/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,vfonov/ITK,vfonov/ITK,LucasGandel/ITK,LucasGandel/ITK,vfonov/ITK,LucasGandel/ITK,vfonov/ITK,richardbeare/ITK,Kitware/ITK,richardbeare/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,thewtex/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,malaterre/ITK,Kitware/ITK,BRAINSia/ITK,hjmjohnson/ITK