[llvm] [AArch64][SME2] Fix mnemonic in uqcvt-diagnostics.s test (NFC) (PR #200824)

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

@jthackray you forgot to approve (assuming you meant to do so given LGTM)

https://github.com/llvm/llvm-project/pull/200824