Doc: `ResourceManager::instance()` returns reference, not pointer.

Maybe a remnant of old API.

Signed-off-by: Squareys <Squareys@googlemail.com>

src/Magnum/DebugTools/ShapeRenderer.h

@@ -126,7 +126,7 @@ Visualizes collision shapes using wireframe primitives. See
 Example code:
 @code
 // Create some options
-DebugTools::ResourceManager::instance()->set("red",
+DebugTools::ResourceManager::instance().set("red",
     DebugTools::ShapeRendererOptions().setColor({1.0f, 0.0f, 0.0f}));
 
 // Create debug renderer for given shape, use "red" options for it

src/Magnum/ResourceManager.h

@@ -231,7 +231,7 @@ Resource<Mesh> cube{manager.get<Mesh>("cube")};
 if(!cube) {
     Mesh* mesh = new Mesh;
     // ...
-    manager->set(cube.key(), mesh, ResourceDataState::Final, ResourcePolicy::Resident);
+    manager.set(cube.key(), mesh, ResourceDataState::Final, ResourcePolicy::Resident);
 }
 @endcode
 -   Using the resource data.