add missing end backslash to multi-line curl commands

[Alys]

Adjusts the two example curl commands shown below so that they have a backslash at the end of the first line.

I have NOT tried to compile this because so much effort for such a tiny change, but shout if you want me to before considering this for merging.

https://api.beeminder.com/#postdata "Create a datapoint"

https://api.beeminder.com/#postdatas "Create multiple datapoints"

[dreeves]

http://beeminder.com/changelog#4209