rectifies sprintf problem with percentage in parts

If within a 'part' a percentage symbol '%' is placed and referenced later by `$part_content` in the export routine, it acts as an unintentional placeholder for a formatted string when passed to `sprintf()`. This results in wiping out all content in the part during the export. 

By modifying how we pass part content to `sprintf()` allows for percentage symbols in the content area to make it through export routines. 

Note: this PR makes it similar to how content is passed to `sprintf()` in chapters.