gtksourceview/template: align description with other themes

[trueNAHO]

Align the description with the /modules/discord/common/theme-header.nix
and /modules/vscode/package.json themes.

Here are the relevant theme extracts:

$ rg 'Theme configured via NixOS or Home Manager.'
modules/discord/common/theme-header.nix:  * @description Theme configured via NixOS or Home Manager.
modules/gtksourceview/template.xml.mustache:  <_description>Theme configured via NixOS or Home Manager.</_description>
modules/vscode/package.json:  "description": "Theme configured via NixOS or Home Manager.",

---

• I certify that I have the right to submit this contribution under the MIT license
• Commit messages adhere to Stylix commit conventions
• Theming changes adhere to the Stylix style guide
• Changes have been tested locally
• Changes have been tested in testbeds
• Each commit in this PR is suitable for backport to the current stable branch