[SPARK-32554][K8S][DOCS] Remove the words "experimental" in the k8s document
[SPARK-32554][K8S][DOCS] Remove the words "experimental" in the k8s document

### What changes were proposed in this pull request?

This PR targets at dropping the words "experimental" in the k8s document from the primary branch.

This update comes from a thread in the spark-dev mailing list: http://apache-spark-developers-list.1001551.n3.nabble.com/spark-on-k8s-is-still-experimental-td29942.html

### Why are the changes needed?

To prepare a GA announcement for the k8s scheduler in the next feature release (v3.1.0)

### Does this PR introduce _any_ user-facing change?

Yes

BEFORE:
<img width="938" alt="Screen Shot 2020-08-10 at 21 17 48" src="https://user-images.githubusercontent.com/692303/89781831-0752fd00-db4f-11ea-843a-67fb23fc8f71.png">
AFTER:
<img width="874" alt="Screen Shot 2020-08-10 at 21 17 21" src="https://user-images.githubusercontent.com/692303/89781816-01f5b280-db4f-11ea-9ab4-4d1012bad80e.png">

### How was this patch tested?

N/A

Closes #29368 from maropu/UpdateDocForK8S.

Authored-by: Takeshi Yamamuro <9770f4f4b25a32d3dfa34670a10767defeb974f3@apache.org>
Signed-off-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>

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