Update image reference in Zscaler ZIA README.

[vinit-chauhan]

What does this PR do?

The image in the README file was not visible on the GitHub page. This PR updates the image file reference in the README file.

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

• [ ]

How to test this PR locally

• Clone integrations repo.
• Install elastic-package locally.
• Start elastic stack using elastic-package.
• Move to integrations/packages/zscaler_zia directory.
• Run the following command to run tests.

elastic-package build
elastic-package stack up -d -s package-registry

Related issues

Screenshots

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Build stats

• Start Time: 2022-04-11T07:04:26.101+0000

• Duration: 17 min 53 sec

Test stats 🧪

Test	Results
Failed	0
Passed	60
Skipped	0
Total	60

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