echo 'Nothing so needs reforming as other people's habits.'
echo 'Nothing so needs reforming as other people'\''s habits.'
echo "Nothing so needs reforming as other people's habits."
When writing a string in single-quotes, you have to make sure that any apostrophes in the text don't accidentally terminate the single-quoted string prematurely.
Escape them properly (see the correct code) or switch quotes to avoid the problem.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.