Added type_conversion and strings to matrix topics.

config.json

       "difficulty": 4,
       "slug": "matrix",
       "topics": [
-	"loops",
-	"arrays",
-	"pattern_matching",
-	"integers"
-	],
+        "loops",
+        "arrays",
+        "pattern_matching",
+        "integers",
+        "strings",
+        "type_conversion"
+      ],
       "unlocked_by": null,
       "uuid": "c1d4e0b4-6a0f-4be9-8222-345966621f53"
     },