docs(responseInterceptor): fix header manipulation example

[mcgwiz]

Description

responseInterceptor expects its function argument to return a value that can be passed to Buffer.from.

Motivation and Context

The documentation is misleading.

How has this been tested?

Confirmed locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

coverage: 97.619%. remained the same
when pulling e9ab06f on mcgwiz:patch-1
into 84bfa46 on chimurai:master.

[chimurai]

Thanks!