update Palo Alto Cortex XDR readme

[ellis-elastic]

updated the links in the file to Palo Alto Cortex XDR documentation.

Type of change

• Enhancement

What does this PR do?

updated the links in the file to Palo Alto Cortex XDR documentation.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

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• Start Time: 2022-06-13T14:31:01.174+0000

• Duration: 15 min 2 sec

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Passed	6
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Total	6

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚 3.432
Classes	100.0% (1/1)	💚 3.432
Methods	100.0% (14/14)	💚 11.111
Lines	89.655% (390/435)	👎 -0.237
Conditionals	100.0% (0/0)	💚

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