Update 03-assertions.md because of issue #2624

[JigmetAvinash]

Adding text in assertion planning in order to inform about ava assertion not stopping even when they fail linked to issue
#2624 Document that awa assertions don't throw |

I also have 3 of my friends using this tool, I also do. I was thinking why does it not stop and found an issue already. This minor change will help the users very much

[novemberborn]

Thanks for the PR @ChopCodes!

This paragraph is about assertion planning, so it'd be the wrong place to add this clarification.

However, the behavior has changed in main, see this PR: #3246. Which incidentally also adds the clarification discussed in #2624.

For next time, it can be useful to check when documentation was last changed, through the commit history. That would have made it apparent #2624 can be closed.