update py/unreachable-statement to match javascript/go

Author: erik-krogh

python/ql/src/Statements/UnreachableCode.ql

@@ -61,4 +61,4 @@ predicate reportable_unreachable(Stmt s) {
 
 from Stmt s
 where reportable_unreachable(s)
-select s, "Unreachable statement."
+select s, "This statement is unreachable."