You cant add a link that contains HTML code:
pi@noads:~ $ curl -s https://github.com/Perflyst/PiHoleBlocklist/blob/master/android-tracking.txt | head -20
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<link rel="dns-prefetch" href="https://github.githubassets.com">
<link rel="dns-prefetch" href="https://avatars0.githubusercontent.com">
<link rel="dns-prefetch" href="https://avatars1.githubusercontent.com">
<link rel="dns-prefetch" href="https://avatars2.githubusercontent.com">
<link rel="dns-prefetch" href="https://avatars3.githubusercontent.com">
<link rel="dns-prefetch" href="https://github-cloud.s3.amazonaws.com">
<link rel="dns-prefetch" href="https://user-images.githubusercontent.com/">
You can add the "Raw" link:
https://raw.githubusercontent.com/Perflyst/PiHoleBlocklist/master/android-tracking.txt
pi@noads:~ $ curl -s https://raw.githubusercontent.com/Perflyst/PiHoleBlocklist/master/android-tracking.txt | head -20
# Created with:
# LineageOS 14.1
# Android 6 on Huawei
# Android 8 OnePlus
# iPhone 6
# Facebook
graph.facebook.com
b-graph.facebook.com
a-graph.facebook.com
graph.instagram.com
b-api.facebook.com
edge-mqtt.facebook.com
mqtt-mini.facebook.com
# Crashlytics
e.crashlytics.com
reports.crashlytics.com
settings.crashlytics.com