sprintfをsnprintfに置き換え

[shogo82148]

警告が出ていたので修正

./utils.h:47:8: warning: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Wdeprecated-declarations]
  std::sprintf(s, "%-16f", val);