[dart/en] Fix logical mistake

[ppmpreetham]

• I solemnly swear that this is all original content of which I am the original author
• Pull request title is prepended with [language/lang-code] (example [python/fr-fr] or [java/en])
• Pull request touches only one file (or a set of logically related files with similar changes made)
• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md
  • Yes, I have double-checked quotes and field names!

[vendethiel]

Thanks! I tweaked the wording a bit, I'm not sure the number of line is that important :).