Fix typo in Readme, put -> put_file

Description

Brand new to this crate and was trying the sample code and had an error.

Fixes typo in example code in the README.

no method named `put` found for struct `ImplFtpStream`

Please select relevant options.

[x] 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)
[ ] This change requires a documentation update

[x] My code follows the contribution guidelines of this project
[x] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[x] My changes generate no new warnings
[ ] I formatted the code with cargo fmt
[ ] I linted my code using cargo clippy and reports no warnings
[ ] I have added tests that prove my fix is effective or that my feature works
[ ] I have introduced no C-bindings
[ ] I increased or maintained the code coverage for the project, compared to the previous commit

wait for a project maintainer to fulfill this section...