[2386] Update exception message

src/lib/dns/rdata/generic/detail/nsec_bitmap.cc

@@ -150,7 +150,7 @@ buildBitmapsFromLexer(const char* const rrtype_name,
 
     if (!have_rrtypes) {
          isc_throw(InvalidRdataText,
-                   rrtype_name << " record does not end with RR type string");
+                   rrtype_name << " record does not end with RR type mnemonic");
     }
 
     for (int window = 0; window < 256; ++window) {