dt-bindings: riscv: fix the schema compatible string for the HiFive Unleashed board

The YAML binding document for SiFive boards has an incorrect
compatible string for the HiFive Unleashed board.  Change it to match
the name of the board on the SiFive web site:

   https://www.sifive.com/boards/hifive-unleashed

which also matches the contents of the board DT data file:

   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dts#n13



Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Acked-by: Rob Herring <robh@kernel.org>