Both are correct; content is the singular and contents is the plural noun.
The confusion arises because the singular and the plural forms have different meanings:
- The plural form 'contents' as an uncountable noun as a Word for the things that are inside something such as a box, bottle, building, or room (The contents of the cup spilled all over my homework.) and a list at the beginning of a book or magazine, showing the parts into which the book or magazine is divided (You'll find it in the table of contents.).
- The singular form 'content' as an uncountable noun as a Word for the subject, ideas, or story that a piece of writing, a radio or television program, or website deals with (The movie has content not suitable for children.) or the proportion of a specified substance (This cereal has a high sugar content.); and a Word for a feeling of happiness and satisfaction (We're enjoying the content of the lakeside on our vacation.)
The Word 'content' is also a verb and an adjective.
