fix: typo in readme (#712)

dist/ncc/buildin/readme.md

@@ -2,6 +2,6 @@
 
 This directory will contain the webpack built-ins, like
 `module.js`, so that they can be accessed by webpack when
-it's being executeed inside the bundled ncc file.
+it's being executed inside the bundled ncc file.
 
 These files are published to npm.