The issue is the akismet developers changed the read.txt file to indicate the plugin is compatible up to WP 4.0 (in anticipation of the release of WP 4.0) but did not do a plugin update. So until they do, Wordfence will flag it as a file discrepancy in your files.
You can either update the plugin manually or ignore it until the plugin is updated. I chose to ignore it for now.