Fix typo in CONTRIBUTING
Found via codespell -q 3 -S ./desktop/org.kde.spectacle.appdata.xml,./desktop/org.kde.spectacle.desktop.cmake -L nd,splitted,taget
Found via codespell -q 3 -S ./desktop/org.kde.spectacle.appdata.xml,./desktop/org.kde.spectacle.desktop.cmake -L nd,splitted,taget