Some context that may or may not be relevant.
A cloud hosting provider named DigitalOcean hosts an event every year called Hacktoberfest to encourage people to contribute to open source projects in exchange for a T-shirt. Wonderful as it is to encourage more people to get into open source, offering a reward in return for gameable metrics and packing it all into one month resulted in many projects receiving a flood of low-quality pull requests that were perceived by some maintainers as spam. A number of these involved trivial changes or vandalism to README files, some inspired by a demonstration on a YouTube channel.
At worst, we're talking about things like this "improve docs" PR, which only added unnecessary periods to comments, or this one, which added an odd and unnecessary header to a README. It was particularly noticeable this year, with some projects receiving a dozen of these in the first day of the event, which caused a lot of consternation among project maintainers and blog posts like DigitalOcean's Hacktoberfest is Hurting Open Source.
Hacktoberfest is over, and that specific problem has abated (the amount of spam caused them to change the way the program worked after a few days to make it opt-in for project maintainers), at least until next year, but that incident gives a bit of context into how many maintainers think about pull requests: PRs exist to improve the software, and PRs that seem to exist just for the sake of changing something without a clear purpose or benefit may be frowned upon, especially in bulk. If you were only fixing a typo in a comment, seemed to be changing documentation for no real reason, or arbitrarily rewriting a bit of code without making any particular improvement, that's the sort of thing that could raise eyebrows.
But in your case, your PR seems like it would genuinely improve the software. Having multiple typos in a README, assuming they are real typos and not regional spelling differences, is less than ideal, and you're providing a material improvement by fixing them.
The fact that you're asking this question and are concerned with respecting the maintainer's time makes it clear that your intentions are noble. Ultimately, to assuage your concerns, reviewing and merging a simple pull request is a quite quick and painless operation for a maintainer—look at the diff and click a button.
Lastly, GitHub is fundamentally just humans talking to each other. It's completely ok to write something like "This is my first pull request. I hope this is useful, but no worries if it's not" or something to that effect if you're unsure.