[sw/silicon_creator] Increase min HD in error_unittest.cc
[sw/silicon_creator] Increase min HD in error_unittest.cc

This commit increases the value of `kMinimumHammingDistance` to 6 (one
less than the current minimum HD from `kErrorOk`) and removes the FIXME.

Signed-off-by: Alphan Ulusoy <23b245cc5a07aacf75a9db847b24c67dee1707bf@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan