Update docs for XGBoost

[pramodrj07]

1. Add the name of 'XGBoost' crd in title
2. Add command to fetch pods explicitly in 'kubeflow' namespace

These changes are aimed to improve the readability of the XGBoost page
and also to catch up with latest doc changes of others like MXNet,
pytorch

[google-oss-robot]

Hi @pramodrj07. Thanks for your PR.

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[pramodrj07]

/assign @terrytangyuan

[terrytangyuan]

/lgtm

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pramodrj07, terrytangyuan

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