fix(policy): use = instead of ≥ for thresh Display

thresh requires exactly k satisfactions to be valid, not k or more.
Providing too many satisfactions makes the witness invalid.

Addresses apoelstra's review feedback on PR rust-bitcoin#914.

Author: febyeji

src/policy/mod.rs

@@ -289,7 +289,7 @@ mod tests {
         assert_eq!(pol.normalized().to_string(), "(pk(A) ∨ pk(B))");
 
         let pol = SemanticPol::from_str("thresh(2,pk(A),pk(B),pk(C))").unwrap();
-        assert_eq!(pol.normalized().to_string(), "#{pk(A), pk(B), pk(C)} ≥ 2");
+        assert_eq!(pol.normalized().to_string(), "#{pk(A), pk(B), pk(C)} = 2");
     }
 
     #[test]

src/policy/semantic.rs

@@ -280,7 +280,7 @@ impl<Pk: MiniscriptKey> fmt::Display for Policy<Pk> {
                     for sub in iter {
                         write!(f, ", {}", sub)?;
                     }
-                    write!(f, "}} ≥ {}", thresh.k())
+                    write!(f, "}} = {}", thresh.k())
                 }
             }
         }