docs: fix default value of header to be true

[eyalch]

The default value of header in the config object is true, but the docs claim that it's false.

Here's the source code of the actual default value:

PapaParse/papaparse.js

Line 275 in 841e1d4

var _writeHeader = true;

[mees-]

The code reference is to a function that converts JSON to csv. This doesn't mean that when converting from csv to JSON the default is true