fix many typos found by Harald

handout/code/2a/git_status.sh

handout/code/2a/gitignore.txt

handout/code/2c/ruff_format_and_check.sh

handout/code/2c/rununittests_snippet.py

handout/sections/1-intro.typ

handout/sections/1c-hardcoded-information.typ

handout/sections/1d-input-validation.typ

handout/sections/1e-debug.typ

handout/sections/2a-git.typ

handout/sections/2c-testing-and-tools.typ

handout/sections/3a1-credentials.typ

handout/sections/3b-webservices.typ

handout/sections/3d-excel.typ