KVM: Documentation: Fix title underline too short warning

Fix "WARNING: Title underline too short" by extending title line to the
proper length.

Signed-off-by: Chang Yu <marcus.yu.56@gmail.com>
Message-ID: <ZqB3lofbzMQh5Q-5@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: cmkyu

Documentation/virt/kvm/api.rst

@@ -6365,7 +6365,7 @@ a single guest_memfd file, but the bound ranges must not overlap).
 See KVM_SET_USER_MEMORY_REGION2 for additional details.
 
 4.143 KVM_PRE_FAULT_MEMORY
-------------------------
+---------------------------
 
 :Capability: KVM_CAP_PRE_FAULT_MEMORY
 :Architectures: none