doc: improve list of fields covered by sighash

This will result in the rendered numbering being different from the
numbering in the source code, but that's probably better than the
existing situation where the "3b" item does not show up as an
independent list item.

There's no good way I can see to fix this because of limitations of
rustfmt's list facilities. And anyway this is legacy code. So just do
something that makes clippy happy.

src/descriptor/csfs_cov/cov.rs

@@ -25,7 +25,7 @@
 //! 1. nVersion of the transaction (4-byte little endian)
 //! 2. hashPrevouts (32-byte hash)
 //! 3. hashSequence (32-byte hash)
-//! 3b. ELEMENTS EXTRA hashIssuances (32-byte hash)
+//! 3. ELEMENTS EXTRA hashIssuances (32-byte hash)
 //! 4. outpoint (32-byte hash + 4-byte little endian)
 //! 5. scriptCode of the input (serialized as scripts inside CTxOuts)
 //! 6. value of the output spent by this input (8-byte little endian)

src/descriptor/csfs_cov/mod.rs

@@ -26,7 +26,7 @@
 //! 1. nVersion of the transaction (4-byte little endian)
 //! 2. hashPrevouts (32-byte hash)
 //! 3. hashSequence (32-byte hash)
-//! 3b. ELEMENTS EXTRA hashIssuances (32-byte hash)
+//! 3. ELEMENTS EXTRA hashIssuances (32-byte hash)
 //! 4. outpoint (32-byte hash + 4-byte little endian)
 //! 5. scriptCode of the input (serialized as scripts inside CTxOuts)
 //! 6. value of the output spent by this input (8-byte little endian)