I can't find the line you speak of; the code in question goes to about line 14, as I recall, then includes a huge number of lines that are not numbered.
You are looking at the '~juicebox.js' file inside the 'jbcore' folder.
You need to go back a level and find the file named 'Juicebox.jsx' in the same directory as the 'jbcore' folder and the 'readme.html' file.
Once you have found the correct file, you will be able to change line 556 from:
'<juiceboxgallery' + '\n' +
... to:
'<juiceboxgallery galleryTitleHAlign="CENTER" captionHAlign="CENTER" ' + '\n' +
Once you have made this change, these options will be used in all Juicebox galleries subsequently created by the Photoshop plugin.