Change the placeholder reference to use a GitHub PR (#52)

docs/project/goals.md

@@ -7,4 +7,4 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 -->
 
 This is a placeholder. See the
-[pending proposal](https://docs.google.com/document/d/1MJvVIDXQrhIj6hZ7NwMDbDch9XLO2VaYrGq29E57meU/edit).
+[pending proposal](https://github.com/carbon-language/carbon-lang/pull/51/files).