Fix spelling

[bugadani]

Hi! Thank you for helping out with SSD1306 development! Please:

• Check that you've added documentation to any new methods
• Rebase from master if you're not already up to date
• Add or modify an example if there are changes to the public API
• Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, Changed, etc)
• Run rustfmt on the project with cargo fmt --all - CI will not pass without this step
• Check that your branch is up to date with master and that CI is passing once the PR is opened

PR description

Looking at some of the stuff that my spell checker found, it might be set to US Enligsh, so I hope I didn't unfix something that was correct in real English.

[jamwaffles]

Good catch, thanks