release: fix syntax error in verify_rc.sh

[amoeba]

The previous commit didn't fix the issue because \ needs to be the final
character, or at least it does on my system.

Ref: #781 (comment)

[amoeba]

Without this fix, I get,

+ download 'https://www.apache.org/dyn/closer.lua?action=download&filename=arrow/KEYS' KEYS
+ curl --fail --location ' '
curl: (3) URL rejected: Malformed input to a URL function

[zeroshade]

beat me to it, i noticed this when i saw the lint failure. sigh my bad on that one