Remove extraneous apostrophe in "its" (#112)

Author: dangerismycat

docs/react-testing-library/api.md

@@ -159,7 +159,7 @@ default to `document.body`.
 
 This is useful when the component you want to test renders something outside the
 container div, e.g. when you want to snapshot test your portal component which
-renders it's HTML directly in the body.
+renders its HTML directly in the body.
 
 > Note: the queries returned by the `render` looks into baseElement, so you can
 > use queries to test your portal component without the baseElement.