fix sinon.assert link in sandbox.assert doc (#1915)

in the https://sinonjs.org/releases/v6.3.5/sandbox/#sandboxassert section,
`sinon.assert` is linked to ./assertions, which translates to 
https://sinonjs.org/releases/v6.3.5/sandbox/assertions which gives a 404, as
`.` in the url is interpreted to be under `/sandbox/`, while the assertions documentation
is on the same hierarchy level

docs/_releases/v6.3.5/sandbox.md

@@ -124,7 +124,7 @@ sinon.config = {
 
 #### `sandbox.assert();`
 
-A convenience reference for [`sinon.assert`](./assertions)
+A convenience reference for [`sinon.assert`](../assertions)
 
 *Since `[email protected]`*