Fix typing errors

[dforsi]

Most errors are in comments. Only 3 are in messages visible to users:

• src/CubicSDR.h
• src/visual/SpectrumCanvas.cpp
• src/visual/WaterfallCanvas.cpp

Found with: codespell --skip=external --ignore-words-list=axix,bord,parm,soley,ue --write-changes --interactive 2 and a manual edit.

[vsonnier]

Better English doesn't hurt, thanks for your contribution !