Fix typo in SE-0155 (#670)

Author: BasThomas

proposals/0155-normalize-enum-case-representation.md

@@ -14,7 +14,7 @@ pattern matching in several places.
 
 Enums, therefore, can be made more "regular" when we replace tuple as the
 representation of associated case values. This proposal aims to define the
-effect of doings so on various parts of the language.
+effect of doing so on various parts of the language.
 
 Swift-evolution thread: [Normalize Enum Case Representation (rev. 2)][]