Remove the trailing document separator (---) from the generated example manifests

[ulucinar]

What happened?

Currently, the generated example manifests have a trailing --- and when folks make a copy of a generated manifest under to the examples directory of a provider and when the up tooling processes it while building a Crossplane package, this may cause issues.

How can we reproduce it?

As an example, please observe the trailing document separator here. Upjet produces the example manifests when configured to do so.

Notes:

• The trailing document separator and the interim ones are being injected into the stream here: https://github.com/upbound/upjet/blob/645d7260d814cb67db2280e92988051d30774a09/pkg/examples/example.go#L172