To answer that question, we need to give a little more insight into the story. After death hits the Creeds, and with the hesitant guidance of quirky neighbor Jud Crandall (played by Fred Gwynne in the original film, and John Lithgow in this new version), a burial ground located deep in the woods is revealed to Louis (played by Dale Midkiff previously, and Jason Clarke now), the family's patriarch, where, legend has it, the soil surrounding the plot of land has the power to bring the recently deceased back to life. Whether you're familiar with the rest of the story or not, it's easy to deduce that nothing good will come of this discovery.
Creepy kids are a horror trope that Stephen King loves to use again and again, and here in Pet Sematary, it's a gang of young children who originally discovered, and thusly took ownership of, the graveyard in question -- using the land's magic to resurrect recently deceased animals, with the purpose of avoiding the pain that comes with saying goodbye to a beloved pet.
And because these children are the cemetery's caretakers, the signage identifying the holy ground was created by them as well. Young kids don't, uh, typically have a full grasp on proper grammar. So that, in a nutshell, is why we ended up getting Pet Sematary and not "Pet Cemetery." The kids beefed the spelling.