DOC: Consistent variable names (sheetname vs sheet_name, issue 10559)

[terrytangyuan]

See issue #10559. Done testing after the changes.

[jorisvandenbossche]

@terrytangyuan the tests are failing because you changed the keyword names as well in the implementation, where this depends on the libraries used by pandas (xlrd, openpyxl, etc). So you also have to check there what name they are using.

[terrytangyuan]

@jorisvandenbossche Thanks. I tried but failed again. It's hard to deal with those internal usages. Might just leave this task to people who are more familiar with them.