Update getting-started.adoc - fix xref:preparing-boot-media.adoc typo
A fix of a typo in prepare-boot-media.adoc file name of xref.
Should be "backported" also to f40 to make the xref/link working.
A fix of a typo in prepare-boot-media.adoc file name of xref.
Should be "backported" also to f40 to make the xref/link working.