Not quite correct. There are 2 ways for an early election
The first is if two thirds of MPs vote to hold an election. This is a very high bar and would in practice require both Labour and Conservative support.
The second is if there is a no confidence vote in the government of the day. After such a vote other parties are given 14 days to form another government. If none can be formed, a new election is held.
Holding a no confidence vote in yourself is a theoretical possibility , but not a very sensible one
But all May has to do is the first option- how do you object to the policy of a democratically elected government if you have refused to allow the people say on an alternative point of view. It would give May unquestionable authority whilst still being subject to Parliamentary scrutiny (win or lose the vote)
Repealing the Fixed Term Act is of course another alternative, but it is not the only way to bring about the required result
quick ps- she could even use the SNP votes (if they voted against her) to insist on a snap independence referendum in Scotland on her own terms- that I suspect is the last thing the SNP would want without being able to play their "victim card" and rigging the electoral roll