triangle: fix test name

Fixes: #419

exercises/triangle/src/test/java/TriangleTest.java

     public final ExpectedException thrown = ExpectedException.none();
 
     @Test
-    public void equilateralTriangleHaveEqualSides() throws TriangleException {
+    public void equilateralTrianglesHaveEqualSides() throws TriangleException {
         Triangle triangle = new Triangle(2, 2, 2);
 
         assertEquals(TriangleKind.EQUILATERAL, triangle.getKind());