Fix typo in text

[antvaset]

This fixes the typo in the decimal value range and makes it consistent with

cql/spec/09-b-cqlreference.adoc

Line 2419 in 3553f6f

For [.id]#Decimal#, [.kw]#maximum# returns the maximum representable decimal value, (10^28^ – 1) / 10^8^ (99999999999999999999.99999999).

and

cql/spec/03-developersguide.adoc

Line 675 in 3553f6f

|*Decimal* |(-10^28^+1)/10^8^..(10^28^-1)/10^8^ |10^-8^

.