docs: improve PredictTextSentimentAnalysisSample

[wallind]

Description

Updates a code sample to have safe serialization of input which may come unsanitized.

Additional Context

I was referred here by @kweinmeister as I'm told this is the new house of what used to live at https://github.com/googleapis/java-aiplatform.

This PR is duplicate of one there: googleapis/java-aiplatform#1098

Github Ticket(s)

Fixes #7561 ☕️

Checklist

• I have followed Sample Format Guide
• pom.xml parent set to latest shared-configuration
• Appropriate changes to README are included in PR
• API's need to be enabled to test (tell us)
• Environment Variables need to be set (ask us to set them)
• Tests pass: mvn clean verify required
• Lint passes: mvn -P lint checkstyle:check required
• Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• Please merge this PR for me once it is approved.

[wallind]

API's need to be enabled to test (tell us)
Environment Variables need to be set (ask us to set them)

apparently per the checklist I need to ask for you guys to do these things? I didn't mess with anything remotely complex so I can't see how it applies to me but asking none-the-less.

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[wallind]

thanks for the collaboration guys! low key can't wait for my changes to be rendering on a GCP hosted docs page 👨‍💻 . That sweet sweet GCP street cred.

[kweinmeister]

Thanks for your contribution, @wallind!