My doubt arises from the Wikipedia page on the Cauchy integral formula, where the set $D=\{z:|z-z_0|\le r\}$ is defined as an open set.
As it includes also its boundary points $|z-z_0|=r$, shouldn't it be defined a closed set? (while $D=\{z:|z-z_0|\lt r\}$ be an open set?)
José Carlos Santos made a comment instead of an answer, otherwise I would have flagged his response as correct. Indeed, D is never referred as a open set, but as a closed disk contained in the U open subset of the complex plane. My mistake.