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/postgres.md

@@ -128,7 +128,7 @@ can read more about that [here](https://www.postgresql.org/docs/10/kernel-resour
 ### Overview
 
 The script `synapse_port_db` allows porting an existing synapse server
-backed by SQLite to using PostgreSQL. This is done in as a two phase
+backed by SQLite to using PostgreSQL. This is done as a two phase
 process:
 
 1.  Copy the existing SQLite database to a separate location and run