Spring-ai Fix broken Maven coordinates in README.md

This PR fixes broken Maven coordinates in the README.md.

Comment From: markpollack

Thanks, looks like this already got cleaned up somewhere else. I appreciate the PR

Comment From: izeye

@markpollack Thanks for the feedback, but the README.md doesn't seem to have been updated. Due to the change of the group ID on 0.8.0-SNAPSHOT, it should be either org.springframework.ai with 0.8.0-SNAPSHOT or org.springframework.experimental.ai with 0.7.1-SNAPSHOT. Otherwise, one of the following failures would happen:

* What went wrong:
Execution failed for task ':compileJava'.
> Could not resolve all files for configuration ':compileClasspath'.
   > Could not find org.springframework.experimental.ai:spring-ai-openai-spring-boot-starter:0.8.0-SNAPSHOT.
     Required by:
         project :

* What went wrong:
Execution failed for task ':compileJava'.
> Could not resolve all files for configuration ':compileClasspath'.
   > Could not find org.springframework.ai:spring-ai-openai-spring-boot-starter:0.7.1-SNAPSHOT.
     Required by:
         project :

Am I missing something?