It is quite common in selecting the wrong stream, You develop the knowledge in Core HR by studying the materials. Even in Cite HR you will get lots of information in HR. No need to do any certification course. In interviews tell the fact why you have selected the Major project in Marketing & you try to convince them by you have potential in HR activities and express your skills in Core HR. Surely they will agree for the same.
Learn All the HR activities as much as possible to face any kind of interview in HR
I had faced the similar kind of situation at the startn of my career..I had done my major project in marketing whereas i was looking forward to join HR during my placement..I had to take extra initiatives by learning new things about core hr and going through various books on Human Resources..
Soon after i got through in a core HR dept of a 5 star hotel company.Now am working as a Manager - HR Business Partner in a German based Investment Bank in Bangalore.
So,i would suggest you to learn more about core hr actvities rather than thinking about your past mistakes which i dont think would not hamper your career at all.
Manager - HR Generalist
If your project was in Marketing, so be it. The key is you finished your studies and passed with stellar colors. Be proud of who you are. Nothing wrong with approching your goals and dreams from a different direction then others.
Be well and continue on your fantastic journey.