[gleam/en] Fixed grammar mistakes

[itsjzt]

• [ ] I solemnly swear that this is all original content of which I am the original author
• [x] Pull request title is prepended with [gleam-en] (example [python/fr-fr] or [java/en])
• [x] 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
  • [x] Yes, I have double-checked quotes and field names!