Fix typo in i2c documentation

[Jzavrk]

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

• Check that you've added documentation to any new methods
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• Add or modify an example if there are changes to the public API
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PR description

Unharmful typo fix in i2c interface documentation, 0x3C is the default address.

[eldruin]

Nice catch, thank you!