Fix link for SE-0023

Author: DougGregor

README.md

@@ -108,7 +108,7 @@ below:
 * [SE-0016: Adding initializers to Int and UInt to convert from UnsafePointer and UnsafeMutablePointer](proposals/0016-initializers-for-converting-unsafe-pointers-to-ints.md)
 * [SE-0017: Change `Unmanaged` to use `UnsafePointer`](proposals/0017-convert-unmanaged-to-use-unsafepointer.md)
 * [SE-0019: Swift Testing](proposals/0019-package-manager-testing.md)
-* [SE-0023: API Design Guidelines](proposals/0006-apply-api-guidelines-to-the-standard-library.md)
+* [SE-0023: API Design Guidelines](proposals/0023-api-guidelines.md)
 * [SE-0028: Modernizing Swift's Debugging Identifiers (\__FILE__, etc)](proposals/0028-modernizing-debug-identifiers.md)
 * [SE-0029: Remove implicit tuple splat behavior from function applications](proposals/0029-remove-implicit-tuple-splat.md)
 * [SE-0031: Adjusting inout Declarations for Type Decoration](proposals/0031-adjusting-inout-declarations.md)