In official Greek language there is no “sh” sound (like the thick sh sound in “shop”).
This is the Greek Sigma letter :
“σ” (lower case)
“ς” (sigma at the end of the word)
The Greek Sigma sounds just like the English letter “S”, like in the word “sun”.
In certain parts of Greece, local people may pronounce the Sigma with a much thicker sound, that does resemble the English “sh” sound. However this is an idiomatic sound like many other sound variances in different locations of Greece.
In any case, whether the Sigma is pronounced “s” or “sh” in a Greek word, it is the same letter and the meaning of the word does not change.