docs: Fix FVP tarball name

Update the referenced tarball name for the FVP to match the name
downloaded when running wget. Without this change, the tar command will
error if run without modification, due to not finding the tarball that
was just downloaded.

Fixes 289e0ca003ba ("FVP: Use latest version of FVP_Corstone_SSE-300_Ethos-U55 ")

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>