Update new names in index.rst

Signed-off-by: Dragos Iulian Argint <dragosargint21@gmail.com>

Author: dragosargint

Documentation/teaching/so2/index.rst

@@ -45,5 +45,5 @@ Operating Systems 2
    assign1-kprobe-based-tracer.rst
    assign2-driver-uart.rst
    assign3-software-raid.rst
-   assign-chall-pitix.rst
-   assign4-transport-protocol.rst
+   assign4.a-pitix.rst
+   assign4.b-transport-protocol.rst