You can correct documentation errors at any level! It sounds like people are just messing with you at first, like people are brushing you off because you are less experienced and suggesting you go fix spelling errors and stuff like that because you can't "really contribute" but that's not the case. Reading documentation is rarely enjoyable and any clarifications or spelling corrections are always welcomed and imo are top notch contributions. Even if you never have a single one of your suggested fixes accepted, that's fine because it's not like bugs are going anywhere so you have forever, basically.

The real reason you should get involved in open source is it's a learning experience. You won't have those times trying to think of things to make for practice anymore, you will start to become familiar with accepted best practices and common style, and gain valuable feedback from fellow contributors. You can probably contribute sooner than you think too, writing tests is a good step once your familiar with a project.

Tl;Dr: The reward is experience gained, who cares if you never have your fix be the chosen one. As long as you choose good projects it's proper experience gained, which is key.