Fix typo in example command in "Overlay embedded in SIF"

[YShoji-HEP]

Description of the Pull Request (PR):

In the "Overlay embedded in SIF" section, the original command

$ echo "test" | {command} exec /tmp/nginx.sif sh -c "cat > /var/cache/nginx/test"

does not work because an overlay image is mounted readonly by default.
It is fixed as

$ echo "test" | {command} exec --writable /tmp/nginx.sif sh -c "cat > /var/cache/nginx/test"

[DrDaveD]

You're right. I wonder how that slipped through.

Thanks for the contribution. We need all commits done with git commit -s, though, to signify the Developer Certificate of Origin. So please re-create the commit with git reset HEAD^1, git commit -s, and git push -f.

[YShoji-HEP]

Thanks for pointing that out. I'll re-create the commit with -s as requested and force-push shortly.