message: Update features.md
  Just a very minor typo
  (that said: these docs don't actually explain what a Feature group is before using the term, but that's for another issue)
diff: @@ -76,7 +76,7 @@ The usage of features is subject to a few rules:
  individual features.
  * Feature groups are not allowed to cyclically depend on one another.
  * Dev-dependencies cannot be optional.
  -* Features groups can only reference optional dependencies.
  +* Feature groups can only reference optional dependencies.
  * When a feature is selected, Cargo will call `rustc` with `--cfg
  feature="${feature_name}"`. If a feature group is included, it and all of its
  individual features will be included. This can be tested in code via
  