[doc] fix broken link in execution_correctness spec

Link was pointing to TCB.md, when it should be pointing to the
../README.md file.

Closes: diem#7859

Author: John Naulty

specifications/trusted_computing_base/execution_correctness/README.md

@@ -5,8 +5,7 @@
 This specification outlines the design and implementation of Diem Execution Correctness (LEC): a secured service
 dedicated for executing transactions correctly (with the MOVE VM) and used by Consensus in the Diem payment network.
 
-For those
-unfamiliar with the execution flow of TCB, we recommend reading the architecture of [Trusted Computing Base (TCB)](../TCB.md).
+For those unfamiliar with the execution flow of TCB, we recommend reading the architecture of [Trusted Computing Base (TCB)](../README.md).
 
 ## Overview and Architecture