fix wrong return type of callback of map function

types/mathjs/index.d.ts

@@ -1345,7 +1345,7 @@ declare namespace math {
          * the Matrix/array being traversed.
          * @returns Transformed map of x
          */
-        map(x: Matrix | MathArray, callback: ((value: any, index: any, matrix: Matrix | MathArray) => Matrix | MathArray)): Matrix | MathArray;
+        map(x: Matrix | MathArray, callback: ((value: any, index: any, matrix: Matrix | MathArray) => MathType | string)): Matrix | MathArray;
 
         /**
          * Create a matrix filled with ones. The created matrix can have one or

types/mathjs/mathjs-tests.ts

@@ -270,6 +270,13 @@ Matrices examples
 		math.range('2:-1:-3');
 		math.factorial(math.range('1:6'));
 	}
+
+	// map matrix
+	{
+  	math.map([1, 2, 3], function(value) {
+      return value * value;
+	  });  // returns [1, 4, 9]
+	}
 }
 
 /*