Fix swapped variables in write_all_browser_files() (#1)

[SycamoreLeaf]

Variables fasta_fn and motif_descs are passed between three functions as follows:

_browser_files_main() --> write_all_browser_files() --> write_frac_wigs()

The two variables were swapped at both interfaces.

---

_browser_files_main() --> write_all_browser_files():

https://github.com/nanoporetech/tombo/blob/afdb4d87249806d3a171d009f948f5b8558d35a0/tombo/_text_output_commands.py#L457-L461

https://github.com/nanoporetech/tombo/blob/afdb4d87249806d3a171d009f948f5b8558d35a0/tombo/_text_output_commands.py#L322-L324

write_all_browser_files() --> write_frac_wigs():

https://github.com/nanoporetech/tombo/blob/afdb4d87249806d3a171d009f948f5b8558d35a0/tombo/_text_output_commands.py#L383-L386

https://github.com/nanoporetech/tombo/blob/afdb4d87249806d3a171d009f948f5b8558d35a0/tombo/_text_output_commands.py#L95-L97