Use inner_script instead of spk

Inside the `Sh::inner_script` method we call through to underlying
methods for the respective enum variant. Currently we are using `spk`,
we can instead use `inner_script` (which calls `spk`), this makes the
code read more easily.

Author: tcharding

src/descriptor/sh.rs

@@ -243,7 +243,7 @@ impl<Pk: MiniscriptKey + ToPublicKey> Sh<Pk> {
     pub fn inner_script(&self) -> Script {
         match self.inner {
             ShInner::Wsh(ref wsh) => wsh.inner_script(),
-            ShInner::Wpkh(ref wpkh) => wpkh.spk(),
+            ShInner::Wpkh(ref wpkh) => wpkh.inner_script(),
             ShInner::SortedMulti(ref smv) => smv.encode(),
             ShInner::Ms(ref ms) => ms.encode(),
         }