Fix typos in documentation comments

[per1234]

An update to the codespell tool used for spell checking the content of this repository caught some misspelled words in the documentation contents: misspelling of the word "calculates" caused a failure of the spell check run by the repository's continuous integration system:

https://github.com/arduino-libraries/Arduino_Portenta_OTA/runs/8047636055?check_suite_focus=true#step:4:17

Error: ./examples/OTA_Qspi_Flash/OTA_Qspi_Flash.ino:6: verifiy ==> verify
Error: ./examples/OTA_SD_Portenta/OTA_SD_Portenta.ino:6: verifiy ==> verify

I took this as an opportunity to do a comprehensive review and found a couple additional typos.

[github-actions]

Memory usage change @ a84953c

Board	flash	%	RAM for global variables	%
arduino:mbed:envie_m7	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_nicla:nicla_vision	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/OTA_Qspi_Flash flash	%	examples/OTA_Qspi_Flash RAM for global variables	%	examples/OTA_SD_Portenta flash	%	examples/OTA_SD_Portenta RAM for global variables	%	examples/OTA_Usage_Portenta flash	%	examples/OTA_Usage_Portenta RAM for global variables	%
arduino:mbed:envie_m7	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:mbed_nicla:nicla_vision	0	0.0	0	0.0					0	0.0	0	0.0

Click for full report CSV

Board,examples/OTA_Qspi_Flash<br>flash,%,examples/OTA_Qspi_Flash<br>RAM for global variables,%,examples/OTA_SD_Portenta<br>flash,%,examples/OTA_SD_Portenta<br>RAM for global variables,%,examples/OTA_Usage_Portenta<br>flash,%,examples/OTA_Usage_Portenta<br>RAM for global variables,%
arduino:mbed:envie_m7,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:mbed_nicla:nicla_vision,0,0.0,0,0.0,,,,,0,0.0,0,0.0