Add language specifiers for markdown code blocks

Author: regexident

examples/examples/data-channels/README.md

@@ -3,7 +3,7 @@ data-channels is a WebRTC.rs application that shows how you can send/recv DataCh
 
 ## Instructions
 ### Build data-channels
-shell
+```shell
 cargo build --example data-channels
 ```
 

examples/examples/insertable-streams/README.md

@@ -8,7 +8,7 @@ E2E encyption, add metadata or insert a completely different video feed!
 
 ## Instructions
 ### Create IVF named `output.ivf` that contains a VP8 track
-shell
+```shell
 ffmpeg -i $INPUT_FILE -g 30 output.ivf
 ```
 

examples/examples/simulcast/README.md

@@ -7,7 +7,7 @@ is selected.
 
 ## Instructions
 ### Build simulcast
-shell
+```shell
 cargo build --example simulcast
 ```