Documentation: KVM: make corrections to vcpu-requests.rst

Make corrections to punctuation and grammar.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Andrew Jones <drjones@redhat.com>
Cc: Christoffer Dall <cdall@linaro.org>
Cc: kvm@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230612030810.23376-5-rdunlap@infradead.org