Update a reference to upb to point to the new location (#19780)

μpb has moved into the `protobuf` repo on August 25, 2023. Related with fa75cd4.

Closes #19780

COPYBARA_INTEGRATE_REVIEW=#19780 from y-yagi:update_reference_to_upb 3fba4d1
PiperOrigin-RevId: 710163065

ruby/README.md

@@ -51,15 +51,14 @@ puts MyTestMessage.encode_json(mymessage)
 
 Installation from Source (Building Gem)
 ---------------------------------------
-
 
 Protocol Buffers has a new experimental backend that uses the
 [ffi](https://github.com/ffi/ffi) gem to provide a unified C-based
 implementation across Ruby interpreters based on
-[UPB](https://github.com/protocolbuffers/upb). For now, use of the FFI
-implementation is opt-in. If any of the following are true, the traditional
-platform-native implementations (MRI-ruby based on CRuby, Java based on JRuby)
-are used instead of the new FFI-based implementation: 1. `ffi` and
+[UPB](https://github.com/protocolbuffers/protobuf/tree/main/upb). For now, use
+of the FFI implementation is opt-in. If any of the following are true, the
+traditional platform-native implementations (MRI-ruby based on CRuby, Java based
+on JRuby) are used instead of the new FFI-based implementation: 1. `ffi` and
 `ffi-compiler` gems are not installed 2. `PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION`
 environment variable has a value other than `FFI` (case-insensitive). 3. FFI is
 unable to load the native library at runtime.