Update ui.md

Author: kilavvy

src/doc/rustc-dev-guide/src/tests/ui.md

@@ -499,7 +499,7 @@ This directive takes comma-separated issue numbers as arguments, or `"unknown"`:
 - `//@ known-bug: rust-lang/chalk#123456`
   (allows arbitrary text before the `#`, which is useful when the issue is on another repo)
 - `//@ known-bug: unknown`
-  (when there is no known issue yet; preferrably open one if it does not already exist)
+  (when there is no known issue yet; preferably open one if it does not already exist)
 
 Do not include [error annotations](#error-annotations) in a test with
 `known-bug`. The test should still include other normal directives and