hello-world.md: Change username to GitHub handle.

Prakhar2826 · shubhamdhama · commit 08aef9052331 · 2018-01-08T23:44:07.000+05:30
diff --git a/submit-a-pull-request/Prakhar2826/hello-world.md b/submit-a-pull-request/Prakhar2826/hello-world.md
@@ -1,2 +1,2 @@
 Hello world
-I am username. :tada:
+I am Prakhar2826. :tada:
