Fix REST web service parameter docs

[BharatDeva]

Fixes #3277.

This updates the Hop REST execution documentation so the request body option list matches the example directly above it.

The example uses:

"service" : "test"

but the option list described the same field as execute. The Hop Server web service documentation also uses service, so this changes the REST option list to service for consistency.

Validation performed locally:

git diff --check
# clean

Select-String -Path docs\hop-user-manual\modules\ROOT\pages\hop-rest\index.adoc -Pattern '`service`','`execute`','"service" : "test"' -SimpleMatch
# line 254: example contains "service" : "test"
# line 273: option list now documents `service`

I could not run the full mvn clean install apache-rat:check locally because Maven/Java are not installed in this Windows/WSL environment. The change is limited to one AsciiDoc word in the user manual.

• Run mvn clean install apache-rat:check to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• If you have a group of commits related to the same change, please squash your commits into one and force push your branch using git rebase -i.
• Mention the appropriate issue in your description (for example: addresses #123), if applicable.
• I hereby declare this contribution to be licensed under the Apache License Version 2.0, January 2004.
• In any other case, please file an Apache Individual Contributor License Agreement.