README.md error about synchronous calls

[hernan-fernandez]

README.md mention:

"Making a synchronous call to query Textract API is not possible for multi-page PDF documents"

This phrase can be improved, since synchronous calls are not possible in any type of PDF file doesn't matter is this is a single-page or multi-page PDF document.

For reference: https://docs.aws.amazon.com/textract/latest/dg/API_DetectDocumentText.html
Documents for synchronous operations can be in PNG or JPEG format. Documents for asynchronous operations can also be in PDF format.