fix topLeftCell

[UziTech]

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Fixes a bug when calling ->freezePane("B2") without a second argument.

The selected cell will be B3 but should be B2

fixes #389

[UziTech]

Another thing I noticed when writing the PR is that when ->freezePane('A1') is called the Excel doc will save without an error but will error when the Excel doc is opened.

Should there be a check for A1 in the freezePane function?

[PowerKiKi]

Thanks, I merged it.

For the ->freezePane('A1'), I don't get any errors when opening with LibreOffice, so I would tend to do nothing.