This looks very similar to a phrase found in section 453a of Plato's "Gorgias" dialogue, where Socrates leads Gorgias, a famous rhetorician and public speaker, through a series of philosophical questions about the nature and morality of rhetoric. However, in that dialogue, the exact wording spoken by Socrates is:
πειθοῦς δημιουργός ἐστιν ἡ ῥητορική
The only difference to what you've written and what Plato wrote is the verb form. So whereas είναι is the infinitive present active form the verb "to be", the word ἐστιν is conjugated for the 3rd person, singular.
Your phrase says, "Rhetoric to be a maker of persuasion". This sounds a little rough on the ears in English, and indeed it does appear grammatically inaccurate in the Greek you've written: πειθούς δημιουργός είναι ἡ ρητορική.
Otherwise, nice job recognizing that contracted feminine, singular, (objective) genitive: πειθούς, although it should have a circumflex above the u-psilon instead of an acute accent.
And by the way δημιουργός has other possible definitions such as "craftsman", "framer", "maker", or "practitioner". For my money, I think that because of the poetic way Socrates has phrased this definition of rhetoric, the funnest way to translate it would be to add a little alliteration by saying, "Rhetoric is a producer of persuasion."