Fix rustdocs build warnings

Building the docs throws a bunch of warnings of form

  warning: unclosed HTML tag ...

Add code ticks to remove the warnings.

bitcoin/src/amount.rs

@@ -1409,7 +1409,7 @@ pub mod serde {
         }
 
         pub mod opt {
-            //! Serialize and deserialize [Option<Amount>] as JSON numbers denominated in BTC.
+            //! Serialize and deserialize `Option<Amount>` as JSON numbers denominated in BTC.
             //! Use with `#[serde(default, with = "amount::serde::as_btc::opt")]`.
 
             use serde::{Deserializer, Serializer, de};

bitcoin/src/blockdata/witness.rs

@@ -29,7 +29,7 @@ use crate::util::taproot::TAPROOT_ANNEX_PREFIX;
 ///
 #[derive(Clone, Default, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
 pub struct Witness {
-    /// Contains the witness Vec<Vec<u8>> serialization without the initial varint indicating the
+    /// Contains the witness `Vec<Vec<u8>>` serialization without the initial varint indicating the
     /// number of elements (which is stored in `witness_elements`)
     content: Vec<u8>,
 

bitcoin/src/psbt/map/input.rs

@@ -113,7 +113,7 @@ pub struct Input {
     pub hash256_preimages: BTreeMap<sha256d::Hash, Vec<u8>>,
     /// Serialized schnorr signature with sighash type for key spend.
     pub tap_key_sig: Option<SchnorrSig>,
-    /// Map of <xonlypubkey>|<leafhash> with signature.
+    /// Map of `<xonlypubkey>|<leafhash>` with signature.
     #[cfg_attr(feature = "serde", serde(with = "crate::serde_utils::btreemap_as_seq"))]
     pub tap_script_sigs: BTreeMap<(XOnlyPublicKey, TapLeafHash), SchnorrSig>,
     /// Map of Control blocks to Script version pair.