Improve example AbslUnparseFlag().

1. Move default case outside switch.  This is a good practice because it allows -Wswitch to warn about missing enum values.

2. Use absl::StrCat() instead of (nonexistent) SimpleItoa().

absl/flags/marshalling.h

@@ -108,8 +108,8 @@
 //    switch (mode) {
 //      case kPlainText: return "plaintext";
 //      case kHtml: return "html";
-//     default: return SimpleItoa(mode);
 //    }
+//    return absl::SimpleItoa(mode);
 //  }
 //
 // Notice that neither `AbslParseFlag()` nor `AbslUnparseFlag()` are class