Fix semver breaking Display change of ChildNumber

Fixes rust-bitcoin#608. In rust-bitcoin#567 the Display impl for ChildNumber was
consciously changed, assuming the semver break would not
affect any correctly implemented downstream projects. We
were wrong.

src/util/bip32.rs

@@ -172,7 +172,7 @@ impl fmt::Display for ChildNumber {
             ChildNumber::Hardened { index } => {
                 fmt::Display::fmt(&index, f)?;
                 let alt = f.alternate();
-                f.write_str(if alt { "'" } else { "h" })
+                f.write_str(if alt { "h" } else { "'" })
             },
             ChildNumber::Normal { index } => fmt::Display::fmt(&index, f),
         }