Add missing alt text for img.

gitstats

 		f.write('</dl>\n')
 
 		f.write(html_header(2, 'Lines of Code'))
-		f.write('<img src="lines_of_code.png">')
+		f.write('<img src="lines_of_code.png" alt="Lines of Code">')
 
 		fg = open(path + '/lines_of_code.dat', 'w')
 		for stamp in sorted(data.changes_by_date.keys()):