[set-theory/fr] Fix multiple errors. (#4361)

Multiple errors prevented the translation from being displayed:
  * contributors was indented with a tab instead of two spaces.
  * the category was not the same as the original english file.
  * a code block was closed at the end of the file, but never
    opened.

fr-fr/set-theory-fr.html.markdown

@@ -1,11 +1,11 @@
-```
 ---
-category: tool
-lang: fr-fr
+category: Algorithms & Data Structures
 name: Set theory
+lang: fr-fr
 contributors: 
   - ["kieutrang", "https://github.com/kieutrang1729"]
 ---
+
 La théorie des ensembles est une branche des mathématiques qui étudie les ensembles, leurs opérations et leurs propriétés.
 
 * Un ensemble est une collection d'éléments disjoints.
@@ -130,5 +130,3 @@ Le produit cartésien de deux ensembles `A` et `B` est l'ensemble contenant tous
 ```
 A × B = { (x, y) | x ∈ A, y ∈ B }
 ```
-
-```