fix calculation of improvement percentage

Author: danez

.changeset/tricky-shoes-check.md

@@ -8,4 +8,4 @@ This is one of the big changes in this new version of react-docgen. It made the
 because there are now finally working TypeScript types for the ASTs.
 
 Another benefit from this change that react-docgen is now a lot faster. 🚀 In some
-tests an improvement of nearly 200% was seen in comparison to version 5.
+tests an improvement of nearly 50% was seen in comparison to version 5.