I know the sound is different from the sound created by real pianist.
Just to clarify, it still sounds good! My thought was merely that if you were interested, it might be a fun experiment to tweak things in little ways to see how it affects the sound - not necessarily to make it sound like a ‘real pianist’
- maybe even just by adding some small fx, or changing the volume of notes or whole phrases over time, things like that!
Can you suggest some tutorial to learn this sound design.
In Sonic Pi - Tutorial, I have found how to use the " changing the synth envelope, adding dynamics, or fx".
Oh, are you saying you have already found these kinds of topics in the Sonic Pi tutorial? That was pretty much still going to be my first recommendation
(see below:)
Sonic Pi’s tutorial is still a fine start 
There’s ‘Duration with envelopes’ - useful if it’s interesting to make notes individually fade in or out over time, or do things like make them staccato notes (notes that are short, sharp and detached from others), etc: (Keeping in mind that you cannot lengthen notes made with the :piano
synth, only shorten them)
https://sonic-pi.net/tutorial#section-2-4
See ‘Additive Synthesis’ and ‘Subtractive Synthesis’, useful if you want to add extra layers of frequencies into each note, or muffle various frequencies, to change the quality of their sound - (basically by playing several synths at the same time to mix their sound, or wrapping synths in one or more filter fx to cut some of their frequencies out):
https://sonic-pi.net/tutorial#section-A-18
https://sonic-pi.net/tutorial#section-A-19
There’s also things like using control
to change the sound of a synth (or an fx that you might have wrapped around it) while it is still running: (say if you wanted to change the volume of a section of notes like you were performing a crescendo etc):
https://sonic-pi.net/tutorial#section-7
For more general, non-Sonic Pi sound design tutorials - an interesting and interactive one, based around synthesiser sounds, and a great one that starts from the complete basics and works its way up, is Learning Synths: