fix: fix log message

The log message was wrong. It's about the hash of the concatenated
`comm_c` and `comm_r_last`.

This was brought up at
#1308 (comment)

storage-proofs/post/src/fallback/vanilla.rs

@@ -506,7 +506,7 @@ impl<'a, Tree: 'a + MerkleTreeTrait> ProofScheme<'a> for FallbackPoSt<'a, Tree>
                     &comm_r_last,
                 )) != AsRef::<[u8]>::as_ref(comm_r)
                 {
-                    error!("comm_c != comm_r_last: {:?}", sector_id);
+                    error!("hash(comm_c || comm_r_last) != comm_r: {:?}", sector_id);
                     return Ok(false);
                 }