image-layout: Fix "auxilary" -> "auxiliary" typo

From 0556a6b (image-layout: ./refs/ -> index.json, 2017-01-26, opencontainers#553).

Signed-off-by: W. Trevor King <wking@tremily.us>

image-layout.md

@@ -198,7 +198,7 @@ Those tags will often be represented in an image-layout repository with matching
 }
 ```
 
-This illustrates an index that provides two named manifest references and an auxilary mediatype for this image layout.
+This illustrates an index that provides two named manifest references and an auxiliary mediatype for this image layout.
 
 
 [descriptors]: ./descriptor.md