changed README to reference _version.py instead of scadnano_version.py

README.md

@@ -51,7 +51,7 @@ Once Python is installed, there are two ways you can install the scadnano Python
     * *required*: [scadnano.py](https://raw.githubusercontent.com/UC-Davis-molecular-computing/scadnano-python-package/master/scadnano/scadnano.py) 
     * *optional*: [modifications.py](https://raw.githubusercontent.com/UC-Davis-molecular-computing/scadnano-python-package/master/scadnano/modifications.py); This contains some common DNA modifications such as biotin and Cy3. 
     * *optional*: [origami_rectangle.py](https://raw.githubusercontent.com/UC-Davis-molecular-computing/scadnano-python-package/master/scadnano/origami_rectangle.py); This can help create origami rectangles, but it is not necessary to use scadnano.
-    * *optional*: [scadnano_version.py](https://raw.githubusercontent.com/UC-Davis-molecular-computing/scadnano-python-package/master/scadnano/scadnano_version.py) This ensures that the current version number is written into any `.dna` files written by the library; otherwise it may be out of date. (Which should not matter for the most part.)
+    * *optional*: [_version.py](https://raw.githubusercontent.com/UC-Davis-molecular-computing/scadnano-python-package/master/scadnano/_version.py) This ensures that the current version number is written into any `.dna` files written by the library; otherwise it may be out of date. (Which should not matter for the most part.)
     
     Unfortunately, the scadnano package uses the Python package [xlwt](https://pypi.org/project/xlwt/) to write Excel files, so in order to call the method [`DNADesign.write_idt_plate_excel_file()`](https://scadnano-python-package.readthedocs.io/#scadnano.scadnano.DNADesign.write_idt_plate_excel_file) to export an Excel file with DNA sequences, xlwt must be installed. To install, type `pip install xlwt` at the command line.