Fix solution to exercise 15 of chapter 3

Author: RamonAraujo

src/chapters/3/sections/named_distributions/problems/15.tex

@@ -1,2 +1,8 @@
-$$F(X = k) = P(X \leq k) = \sum_{i=1}^{k}P(X = i) = \sum_{i=1}^{k} \frac{1}{n} =
-\frac{k}{n}.$$
+\[ F_X(x) = P(X \le x) = \begin{cases}
+        0 &, \text{ if } x < 1\\
+        \frac{\lfloor x \rfloor}{n} &, \text{ if } 1 \le x \le n\\
+        1 &, \text{ if } x > n
+    \end{cases}
+\]
+
+where $\lfloor x \rfloor$ equals the largest integer that is less than or equal to $x$ .