Fix invalid private repository SVG image URLs (#22)

SVG images like the repository hero were not rendered because they used
the URLs for public repositories instead of the ones that allow to
resolve the files in private repositories:

```diff
-https://raw.githubusercontent.com/svengreb/tmpl/master/assets/images/repository-hero.svg?sanitize=true
+https://github.com/svengreb/tmpl/blob/master/assets/images/repository-hero.svg?raw=true
```

Fixes GH-21

README.md

@@ -1,4 +1,4 @@
-<p align="center"><img src="https://raw.githubusercontent.com/svengreb/tmpl/master/assets/images/repository-hero.svg?sanitize=true"/></p>
+<p align="center"><img src="https://github.com/svengreb/tmpl/blob/main/assets/images/repository-hero.svg?raw=true"/></p>
 
 <p align="center"><a href="https://github.com/svengreb/tmpl/releases/latest"><img src="https://img.shields.io/github/release/svengreb/tmpl.svg?style=flat-square&label=Release&logo=github&logoColor=eceff4&colorA=4c566a&colorB=88c0d0"/></a> <a href="https://github.com/svengreb/tmpl/blob/master/CHANGELOG.md"><img src="https://img.shields.io/github/release/svengreb/tmpl.svg?style=flat-square&label=Changelog&logo=github&logoColor=eceff4&colorA=4c566a&colorB=88c0d0"/></a></p>