fix: documentation of snippet cannot be shown when description is list

[ccbhj]

When formating documentation of snippets, we treat the description field of a snippet item as string by default but it could a list of string sometimes(see snippets for shell in friendly-snippets ) which might cause a concatenation failure.

So here is a tiny fix by joining the description list if it is a list when loading snippets, so now it could be shown and looks like this:

Closes #91

[Saghen]

Thanks!