Clarify panic message for too-big shift operations on BitSlices

[rbeard0330]

Thanks for creating this library. I was using this crate in a project of mine, and I got a confusing error message when I tried to shift a BitSlice by an amount equal to its length: cannot shift a bit-slice by more than its length: 100 > 100. This PR just rewords the error message to read: shift must be less than the length of the bit-slice: 100 >= 100.

I also double-checked the docs, and they correctly describe the requirement for the shift operation.

Hope this helps!

[myrrlyn]

Thank you so much!