authorsAlpha cite key corrected to comply with JabRef specification

l[authorsAlpha] cite key separated letters with a '.' JabRef specifies character concatenation

[authorsAlpha]: Corresponds to the BibTeX style "alpha". One author: First three letters of the last name. Two to four authors: First letters of last names concatenated. More than four authors: First letters of last names of first three authors concatenated. "+" at the end.