Avatar size parameter added incorrectly to the URL

**Describe the bug**
The bot adds avatar images with a size parameter, but that's concatenated incorrectly with a duplicated `?`, like this: `https://avatars3.githubusercontent.com/u/1976647?v=4?s=100`.

**To Reproduce**
1. Ask the bot to add a contributor of any kind with `@all-contributors please add @ for `.
2. Observe that the avatar URL is added with a duplicated `?`.

It looks like that this bug has existed forever, our [very first contributor profiles under Orchard Core](https://docs.orchardcore.net/en/latest/docs/community/contributors/) are like this too.

**Expected behavior**
The `s` parameter is added with an ampersand instead: `https://avatars3.githubusercontent.com/u/1976647?v=4&s=100`.

**Screenshots**
Not applicable.

**Additional context**
If it affects anything, this is the page we keep up to date: https://docs.orchardcore.net/en/latest/docs/community/contributors/.