`text_input` example clean up (#24297)

# Objective

Clean up the `text_input` example a bit.

## Solution

- Removed some unneeded style constraints.
- Disable newlines, disable wrapping, and set a responsive width for the
inputs.
- Submit on Enter, not Ctrl + Enter.
- Center the example using an auto margin.

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Co-authored-by: Kevin Chen <chen.kevin.f@gmail.com>