[docs] Change Expo.Constants.exponentVersion to expoVersion for sdk15
[docs] Change Expo.Constants.exponentVersion to expoVersion for sdk15

exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent