You probably didn’t do it intentionally, but you did modify that cell, as Tom points out. You must have had your cursor in the wrong place and pressed backspace at the wrong time and it removed that underscore character from the “channels_last” string.
If you want to be sure there are no other accidentally modifications, you can get a clean copy of the notebook by following the instructions on this thread. Then carefully “copy/paste” over just your completed code from the “YOUR CODE HERE” segments.
Just to make sure we weren’t seeing recent change from the course developers, I went through the exercise of getting the latest clean copy (using the above procedure) and it’s still “channels_last” with the underscore in the latest version.