Update the version of example to 5.15

Update the link of the example code for TLS Socket to 5.15,
as current documentation referes to this version.

Author: Ron Eldor

docs/api/networksocket/TLSSocket.md

@@ -14,7 +14,7 @@ To use secure TLS connections, the application uses the `TLSSocketWrapper` throu
 
 The TLSSocket example creates TLS connection to the HTTPS server and receives a simple response from the server:
 
-[![View code](https://www.mbed.com/embed/?url=https://github.com/ARMmbed/mbed-os-example-tls-socket/blob/master/)](https://github.com/ARMmbed/mbed-os-example-tls-socket/blob/mbed-os-5.14/main.cpp)
+[![View code](https://www.mbed.com/embed/?url=https://github.com/ARMmbed/mbed-os-example-tls-socket/blob/master/)](https://github.com/ARMmbed/mbed-os-example-tls-socket/blob/mbed-os-5.15/main.cpp)
 
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