Update Spherical.d.ts

[spherical.makeSafe()](https://github.com/mrdoob/three.js/blob/950a3df93c9d8d8a6324962139c066eaad654a9e/src/math/Spherical.js#L57) is fluent:

src/math/Spherical.d.ts

@@ -11,7 +11,7 @@ export class Spherical {
 	set( radius: number, phi: number, theta: number ): this;
 	clone(): this;
 	copy( other: Spherical ): this;
-	makeSafe(): void;
+	makeSafe(): this;
 	setFromVector3( v: Vector3 ): this;
 	setFromCartesianCoords( x: number, y: number, z: number ): this;