update-modules.sh should use same path for file check and copy

[viewmagic-chris]

if [ -f "${relative}" ]; then

Should probably read:

    # Look through nearly every file in the cmake-modules repo, but don't copy this file to your project!
    (cd $MODULES_DIR && git ls-files | grep -v "update-modules") | while read -r relative; do
        if [ -f "${MODULES_DIR}/${relative}" ]; then
            echo "${relative}"
            cp "${MODULES_DIR}/${relative}" .
        fi
    done