Documentation changed from variable to file variable type

The documentation changed from use of variable to file variable type.

File variable write the content of value out to a temporary file, and save the path to this file as content of the variable key, so for example $SSH_PRIVATE_KEY would have a value of the temporary path and the temporary file would contain the SSH key saved in the CI/CD file variables settings.

Therefore some code using the variable has to be changed to expect content of variable not to be string but a path to a file.