fix header link

Signed-off-by: Meredith Lancaster <malancas@github.com>

README.md

@@ -18,7 +18,7 @@ for more information on artifact attestations.
 ## Table of Contents
 - [Usage](#usage)
   - [attest](#attest)
-  - [attestProvenance](#attest-provenance)
+  - [attestProvenance](#attestprovenance)
   - [Attestation](#attestation)
 - [Sigstore Instance](#sigstore-instance)
 - [Storage](#storage)