k8s example: updated elasticsearch's image tag from the default 'latest' to '7.4.0'
k8s example: updated elasticsearch's image tag from the default 'latest' to '7.4.0'

To fix the ImagePullBackOff error

Fixes #3401

weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope