Fix mysql dialect text docstring, length is interpreted as byte size

Fix mysql dialect text docstring, length is interpreted as byte size

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### Description
The `Text` and its variant types in MySQL are bytes size limited, not character length, so fixing the doctoring where the upper limit uses the `characters` as the unit instead of `bytes`

https://dev.mysql.com/doc/refman/5.7/en/storage-requirements.html
https://dev.mysql.com/doc/refman/8.0/en/storage-requirements.html

<img width="878" alt="Screenshot 2024-02-15 at 17 27 59" src="https://github.com/sqlalchemy/sqlalchemy/assets/5219229/29731769-f57e-46f9-858b-46feda0ae83c">

### Checklist
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This pull request is:

- [x] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #11018
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/11018
Pull-request-sha: 13fa52917efea9a229c7abf19a3be40e24a79cb9

Change-Id: Iea903a6dc4b52ee4b7b5d2d64256c69abbd1f8aa
(cherry picked from commit 1c58fe53b6fd069cbb82955ddaf9eb5405076146)