Fix wrong use of hBBodyHash in CHAINHEAD in the formal spec (#1138)

In the definition of `CHAINHEAD`, the expression $`\textsf{hBBodyHash}
\; bhb`$ must be changed to $`\textsf{headerHash} \; bh`$.

docs/formal-spec/chain.tex

@@ -924,7 +924,7 @@ \subsection{$\mathsf{CHAINHEAD}$ Transition}
               \eta_c' \\
         \end{array}\right)}
       } \\~\\~\\
-      \var{lab'}\leteq (\hBBlockNo{bhb},~\var{s},~\hBBodyHash{bhb} ) \\
+      \var{lab'}\leteq (\hBBlockNo{bhb},~\var{s},~\headerHash{bh} ) \\
     }
     {
       \var{nes}