GH10559: Minor improvement: Change read_excel sheet name
modify io/excel.py and relevant docs (io.rst) to use sheet_name for read_excel
but allow sheetname to still be used for backwards compatibility.  add test_excel
to verify that sheet_name and sheetname args produce the same result.

 - [X] closes #10559 
 - [X] tests added / passed
 - [X] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry
