Update getAuthorFullName function to include space

Add space between first and last name to match format as taught in "Strings - Concatenation" chapter, and for readability

Here is the [exercise from the Concatenation section](https://github.com/GitbookIO/javascript/commit/5bfd4cbaec436b44b32d2d8b54e9b0b837672209#diff-f8878daa61e33da02738b3b427a48525R24) for reference: ```var fullName = firstName + " " + lastName;```