Fix undefined variable in README example code

[hobostay]

Problem

The example code in the "Measure a paragraph's height" section (line 28) used an undefined variable textWidth:

const { height, lineCount } = layout(prepared, textWidth, 20)

This would cause a ReferenceError: textWidth is not defined if someone tries to run this code directly.

Solution

Changed the undefined variable to a concrete value (320) to match the other examples in the documentation that use 320px as the max width:

const { height, lineCount } = layout(prepared, 320, 20)

This makes the example code immediately runnable without modification.

Testing

The fix is purely documentation-only. No code changes were made.

[chenglou]

Thanks! I folded this into cca3f1d and credited you as co-author.