[Gardening] remove extra "to let" from documentation of Product.library(name:type:targets:) (#3253)

Author: WowbaggersLiquidLunch

Sources/PackageDescription/Product.swift

@@ -139,7 +139,7 @@ public class Product: Encodable {
     /// - Parameters:
     ///     - name: The name of the library product.
     ///     - type: The optional type of the library that's used to determine how to link to the library.
-    ///         Leave this parameter unspecified to let to let the Swift Package Manager choose between static or dynamic linking (recommended).
+    ///         Leave this parameter unspecified to let the Swift Package Manager choose between static or dynamic linking (recommended).
     ///         If you don't support both linkage types, use `.static` or `.dynamic` for this parameter. 
     ///     - targets: The targets that are bundled into a library product.
     public static func library(