[docs] Missing example

[hamza-shezad]

Reference:

mergeable/docs/validators/stale.rst

Line 21 in 590a40f

This is a special use case. The schedule event runs on an interval. When used with stale, it will search for issues and/or pull request that are n days old. See a full example »

Not sure if "See a full example" is referring to the example above it. If not, please add the example to complete the documentation.