Fix createPortal link in API docs (#11000)

The anchor name is lower case

docs/reference-react-dom.md

@@ -16,7 +16,7 @@ The `react-dom` package provides DOM-specific methods that can be used at the to
 - [`hydrate()`](#hydrate)
 - [`unmountComponentAtNode()`](#unmountcomponentatnode)
 - [`findDOMNode()`](#finddomnode)
-- [`createPortal()`](#createPortal)
+- [`createPortal()`](#createportal)
 
 ### Browser Support
 
@@ -107,4 +107,4 @@ If this component has been mounted into the DOM, this returns the corresponding
 ReactDOM.createPortal(child, container)
 ```
 
-Creates a portal. Portals provide a way to [render children into a DOM node that exists outside the hierarchy of the DOM component](/docs/portals.html).
+Creates a portal. Portals provide a way to [render children into a DOM node that exists outside the hierarchy of the DOM component](/docs/portals.html).