Per MOS
I'm keying off the very first bullet point: "If possible, an article title is the subject of the first sentence of the article".  The article name is "Breast", and so the subject of first sentence should match.  As others have pointed out, the subject is "Breast" not "Breasts", a subtle yet distinct difference.  I'd also point to WP:CON, your change doesn't appear to have that either.   †