Update xuartlite_intr_example.c

[ChatterjeePracheta]

In the previous code the Recv and Send commands were not placed properly but the code was running as uartlite ip is an rtos( it tries to execute tasks with same priority parallely). Also the ASCII values below 'A' are not printed properly in vitis serial terminal or tera term. So in this update Recv and Send commands are placed in proper sequential manner for better understanding and also documentation is also changed a bit keeping that in mind. The sendbuffer initial value now starts from A so that all the characters are printed properly.

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