[hotfix] [docs] Fix typos

[uce]

Really helpful documentation! Yesterday, I read over most of the docs and found a few typos, including some typos in code. Feel free to push back on any of the changes that seem unnecessary to you.

A few nit comments:

• In the project setup page, we state that mvn clean package produces an uber JAR which is not true. Is the intention to provide a Maven configuration that does create an uber JAR or are the docs mistaken?
• In the Python walk-through, we first write a function without a known type and then show how to specify a type. I'm wondering we should only show the known type example and keep it as a note that it's also possible to the use the Any type.
• Is RocksDB a strict requirement or can users configure any state backend? If RocksDB is a requirement, we could consider pointing this out in https://github.com/apache/flink-statefun/blob/8376afa/docs/concepts/logical.md#function-lifecycle (last sentence).
• In the Java SDK, we mix tab and space for code formatting. I think spaces are easier to read in the docs (I understand that the code should follow the apache/flink style guide).
• In the Java SDK async requests section, we could use a stateful match function to reduce some of the boilerplate.
• Overall, message routing still felt a bit mysterious after reading over the docs. I don't know if others agree, but we might want to add more details on this part of the system.

[uce]

@sjwiesman @igalshilman @tzulitai I reverted the Docker image repository change and applied Seth's suggestion for the function types. Can you take another look and merge or close this PR?

[tzulitai]

Thanks @uce, will merge this!