message: Fix README.md.
diff: # C++ Digital Signatures CLI
  -This is a command-line tool that can generate [Digital Signature]
  -(../../../docs/PRIMITIVES.md#digital-signatures) keys, and create and
  -verify digital signatures.
  +This is a command-line tool that can generate
  +[Digital Signature](../../../docs/PRIMITIVES.md#digital-signatures)
  +keys, and create and verify digital signatures.
  It demonstrates the basic steps of using Tink, namely loading key material,
  obtaining a primitive, and using the primitive to do crypto.
  