Fix MCE_CPU_KEEPALIVE_PERIOD_REQ method call documentation

The value returned is dbus_int32_t, not dbus_uint32_t. Documentation
changed to match implementation.

[mce-headers] Fix MCE_CPU_KEEPALIVE_PERIOD_REQ method call documentation
Merge pull request #4 from spiiroin/fix_doc

Fix MCE_CPU_KEEPALIVE_PERIOD_REQ method call documentation