Fix minor bugs in `CHAIN` of Shelley formal spec

The following errors must be corrected:

1. In Figure 73, the type of $b_\ell$ must be changed from $\textsf{Slot}$ to $\textsf{BlockNo}$.
2. In Figure 74, the first → in the signature of $\textsf{prtlSeqChecks}$ must be changed to $\in$.
3. In Figure 75, the expression $`\textsf{bhash} \; bhb`$ must be changed to $`\textsf{bhHash} \; bh`$.