update path to built program in README (#52)

Following the 'Building Manually' section of the README:

- https://github.com/keith/reminders-cli#building-manually

I found that the compiled executable wasn't at the path specified.

This PR updates the README to include the proper path.

I'm not sure if this is the canonical default now, or something to do
with the version of `swift` I have installed, so please correct my if
this change isn't universally applicable:

```
⇒  swift --version
swift-driver version: 1.62.15 Apple Swift version 5.7.2 (swiftlang-5.7.2.135.5 clang-1400.0.29.51)
Target: x86_64-apple-macosx13.0
```

README.md

@@ -94,5 +94,5 @@ This requires a recent Xcode installation.
 ```
 $ cd reminders-cli
 $ make build-release
-$ cp .build/release/reminders /usr/local/bin/reminders
+$ cp .build/apple/Products/Release/reminders /usr/local/bin/reminders
 ```