Clarify filenames in README.md

[nidadu]

The current documentation is confusing when calling write_NIRS or write_SNIRF functions in examples in README.md. The variable filename is given for both a .lumo/.lufr file and the output of write_NIRS/write_SNIRF function. Thus, this can lead to overwrite of LUMO files if the code is taken directly from the example.

The update clarifies filenames for write_NIRS and write_SNIRF functions calls:

write_NIRS(filename) -> write_NIRS(nirs_filename) write_SNIRF(filename) -> write_SNIRF(snirf_filename)

[nidadu]

@samuelpowell could you approve this if it looks ok?

[samuelpowell]

OK