specification: Replace hardware RoT with just RoT

In that context the RoT does not have to be entirely in HW, as long as
it's a platform RoT.

Fixes #51

Signed-off-by: Samuel Ortiz <sameo@rivosinc.com>

specification/05-security_model.adoc

@@ -220,7 +220,7 @@ from the following adversaries:
        between a physical device and the untrusted host domain.
      - Although the VMM can tamper with the device IDE extended capabilities,
        the PCIe root port IDE settings must only be available to a TVM TCB
-       component, either the TSM or a hardware root-of-trust.
+       component, either the TSM or a Root-of-Trust.
 
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