Fix more typos

Author: DougGregor

stdlib/public/core/Sendable.swift

@@ -13,7 +13,7 @@
 /// A type whose metatype can be shared across arbitrary concurrent contexts
 /// without introducing a risk of data races. When a generic type `T` conforms
 /// to `SendableMetatype`, its metatype `T.Type` conforms to `Sendable`.  All
-/// concrete types implicitly conform to the SendableMetatype protocol, so its
+/// concrete types implicitly conform to the `SendableMetatype` protocol, so its
 /// primary purpose is in generic code to prohibit the use of isolated
 /// conformances along with the generic type.
 ///