Fix error in ExpressibleByArgument docs (#512)

Author: natecook1000

.gitignore

@@ -5,3 +5,4 @@
 .swiftpm
 .vscode
 .*.sw?
+*.docc-build

Sources/ArgumentParser/Parsable Types/ExpressibleByArgument.swift

@@ -19,7 +19,7 @@ public protocol ExpressibleByArgument {
   /// command-line tool's help screen.
   var defaultValueDescription: String { get }
 
-  /// An array of all possible strings to that can convert to value of this
+  /// An array of all possible strings that can convert to a value of this
   /// type.
   ///
   /// The default implementation of this property returns an empty array.