Kernel: Always format MACAddress with two-byte hex digits in to_string

Author: https://github.com/deoxxa
Commit: fac1148679
Pull-request: https://github.com/SerenityOS/serenity/pull/499
Reviewed-by: https://github.com/awesomekling

Kernel/Net/MACAddress.h

@@ -37,7 +37,7 @@ public:
 
     String to_string() const
     {
-        return String::format("%b:%b:%b:%b:%b:%b", m_data[0], m_data[1], m_data[2], m_data[3], m_data[4], m_data[5]);
+        return String::format("%02x:%02x:%02x:%02x:%02x:%02x", m_data[0], m_data[1], m_data[2], m_data[3], m_data[4], m_data[5]);
     }
 
 private: