docs: clearer variable name in x-bind example

[calebporzio]

Closes #4430

Cherry-picked from #4430 by @sa- with review fix applied (camelCase instead of snake_case).

Summary

• Renames placeholder to placeholderText in the x-bind docs example to avoid confusion between the JS variable name and the HTML placeholder attribute
• Uses camelCase (placeholderText) instead of the original PR's snake_case (placeholder_text) to match JavaScript conventions used throughout Alpine's docs

Original description (from @sa-)

While I was reading through the docs I was quite confused because placeholder was both a variable name and an attribute for the input field. It took me unreasonably long to figure it out and I believe changing the variable name slightly would have helped me grok it faster.