Fix value param in Javadoc of addParamToMultipart

[jjohannes]

This bug causes the javadoc tool to fail for the generated code with:

error: @param argument "obj" is not a parameter name.
   * @param obj Object

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Summary by cubic

Fixes the Javadoc parameter name in the Jersey3 ApiClient template so generated code no longer fails the javadoc tool with @param argument "obj" is not a parameter name.

• Renames the @param tag from obj to value to match the method signature.

^{Written for commit 3940278. Summary will update on new commits.}