docs: migration-guides: release-notes-4.0: fix risc-v typo

It's RISC-V and not RiscV, c.f. https://riscv.org/ (yes, no dash in the
domain name...).

Cc: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: 1a5fd51a50fe39d8095bb5a4f34d6b953e59c58f)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/release-notes-4.0.rst

@@ -152,7 +152,7 @@ New Features / Enhancements in 4.0
       - rust: add support for big endian 64-bit PowerPC
       - rust: Add snapshot checksums for powerpc64le
 
-   - RiscV:
+   - RISC-V:
 
       - libunwind: Enable for rv64
       - systemtap: Enable for riscv64