[1112] Editorial cleanup: added parentheses

src/lib/dns/character_string.cc

@@ -23,7 +23,7 @@ namespace dns {
 
 namespace {
 bool isDigit(char c) {
-    return ('0' <= c) && (c <= '9');
+    return (('0' <= c) && (c <= '9'));
 }
 }