Fix docstring for Format::with_header in arrow-csv

[kylebarron]

Which issue does this PR close?

Closes #.

Rationale for this change

Fix documentation

What changes are included in this PR?

Format::header defaults to false but is documented as defaulting to true

/// The format specification for the CSV file
#[derive(Debug, Clone, Default)]
pub struct Format {
    header: bool,

Are there any user-facing changes?

No

[alamb]

Thank you @kylebarron

Indeed the default is false

/// The format specification for the CSV file
#[derive(Debug, Clone, Default)]
pub struct Format {
    header: bool,
...