Fix `completing-read' call argument

According the `completing-read' docstring, the INITIAL-INPUT parameter has been
deprecated, and the docstring suggested to pass nil for INITIAL-INPUT and supply
the default value DEF instead. In reality, I've been suffering from inconsistent
behavior while using `elisp-slime-nav-describe-elisp-thing-at-point' with Helm
complete. More specifically, when I typed part of a symbol's name, some times
Helm matches the results but some times it doesn't, for no reason. This has
bothered me for a long time, until to the point that I can no longer bear it. So
I decided to either fix the problem, or give up using this package. Fortunately,
immediately after I changed to follow the argument passing protocol suggested in
the `completing-read' docstring, the inconsistency problem went away.
Merge pull request #11 from YorkZ/york

Fix `completing-read' call argument