Update README.md

Fix this error in the installation instructions:

```
go install github.com/google/gnostic/cmd/protoc-gen-openapi
go: 'go install' requires a version when current directory is not in a module
	Try 'go install github.com/google/gnostic/cmd/protoc-gen-openapi@latest' to install the latest version
```

cmd/protoc-gen-openapi/README.md

@@ -6,7 +6,7 @@ Protocol Buffer service.
 
 Installation:
 
-    go install github.com/google/gnostic/cmd/protoc-gen-openapi
+    go install github.com/google/gnostic/cmd/protoc-gen-openapi@latest
 
 Usage:
 
@@ -62,4 +62,4 @@ refers to additional .proto files in the same directory as
             application/json:
               schema:
                 $ref: '#/components/schemas/google.rpc.Status'
-      ```
+      ```