Fix various typos in docs (#17114)

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [X] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

docs/usage/administration/admin_api/background_updates.md

@@ -44,7 +44,7 @@ For each update:
 
 ## Enabled
 
-This API allow pausing background updates.
+This API allows pausing background updates.
 
 Background updates should *not* be paused for significant periods of time, as
 this can affect the performance of Synapse.

docs/usage/administration/admin_faq.md

@@ -241,7 +241,7 @@ in memory constrained environments, or increased if performance starts to
 degrade.
 
 However, degraded performance due to a low cache factor, common on
-machines with slow disks, often leads to explosions in memory use due
+machines with slow disks, often leads to explosions in memory use due to
 backlogged requests. In this case, reducing the cache factor will make
 things worse. Instead, try increasing it drastically. 2.0 is a good
 starting value.