Fixes typo on using form encoding

[dcode]

What does this PR do?

The documentation for using form encoding had a typo on how to specify it. I corrected it to what is listed in the beats source.

Why is it important?

Correct docs make sure users don't have to read the code to guess how it works.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

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Build stats

• Build Cause: Pull request #24329 opened

• Start Time: 2021-03-03T22:00:37.374+0000

• Duration: 20 min 51 sec

• Commit: 355cea1

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[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[leehinman]

🤦 thanks for catching this. Would you mind doing PR against master and I'll backport to 7.12

[marc-gr]

Is this change duplicated in #24336 @leehinman? Maybe we can close this if so if we backport it.

[leehinman]

closing because #24336 covers it.