Fix link to system_description

Signed-off-by: Joachim Strömbergson <joachim@assured.se>

doc/threat_model/threat_model.md

@@ -11,10 +11,10 @@ client app as needed to solve different use cases.
 
 This document describes the threat model for the Tillitis TKey device
 and the device app. Based on [the system
-description](../system_description.md) and use cases, the threat model
-tries to capture and describe the threats that needs to be mitigated
-in order for the device app to work in a secure and trustworthy
-manner.
+description](../system_description/system_description.md) and use
+cases, the threat model tries to capture and describe the threats that
+needs to be mitigated in order for the device app to work in a secure
+and trustworthy manner.
 
 
 ## Assumptions