Update README.md (#735)

Grammatical error.

Author: justfortee

README.md

@@ -1176,7 +1176,7 @@ $ trivy image --skip-dirs "/usr/lib/ruby/gems,/etc" fluent/fluentd:edge
 
 ### Skip update of vulnerability DB
 
-`Trivy` always updates its vulnerability database when it starts operating. This is usually fast, as it is a difference update. But if you want to skip even that, use the `--skip-update` option.
+`Trivy` always updates its vulnerability database when it starts operating. This is usually fast, as it is a different update. But if you want to skip even that, use the `--skip-update` option.
 
 ```
 $ trivy image --skip-update python:3.4-alpine3.9