blockchain_stats: fix sign in formatting function

src/blockchain_utilities/blockchain_stats.cpp

@@ -205,7 +205,7 @@ plot 'stats.csv' index "DATA" using (timecolumn(1,"%Y-%m-%d")):4 with lines, ''
     char buf[8];
     unsigned int i;
     for (i=0; i<24; i++) {
-      sprintf(buf, "\t%02d:00", i);
+      sprintf(buf, "\t%02u:00", i);
       std::cout << buf;
     }
   }