docs(design): Fix typo

Author: epage

DESIGN.md

@@ -33,7 +33,7 @@ Decisions
   - Makes units explicit
   - Can track duration of any operation
 - Defer to cargo to provide test-binary differentiating information, like it does for rustc
-- No equivelant of rustc including a rendered diagnostic
+- No equivalent of rustc including a rendered diagnostic
   - Terse and pretty progress indicators are too nebulous to render (see their notifiers)
   - There is likely not enough value add in the failure message
   - This puts more of a burden on custom test harnesses for their implementation than is strictly needed