Harmonize capitalization

The rest of the chapter uses capitals for set variables

content/sets-functions-relations/sets/basics.tex

@@ -11,8 +11,8 @@
 
 A \emph{set} is a collection of objects, considered {as} a single
 object. The objects making up the set are called \emph{elements} or
-\emph{members} of the set. If $x$ is !!a{element} of a set~$a$, we
-write $x \in a$; if not, we write $x \notin a$. The set which has no
+\emph{members} of the set. If $x$ is !!a{element} of a set~$A$, we
+write $x \in A$; if not, we write $x \notin A$. The set which has no
 !!{element}s is called the \emph{empty} set and
 denoted~``$\emptyset$''.