fix: typo in contract constants example (#239)

Author: PavelVaskou

pages/book/contracts.mdx

@@ -72,7 +72,7 @@ const GlobalConst1: Int = 1000 + ton("42") + pow(10, 9);
 
 contract Example {
     // contract constants are also calculated in compile-time and cannot change
-    const ContractConst1: Int = 2000 + ton("43") pow(10, 9);
+    const ContractConst1: Int = 2000 + ton("43") + pow(10, 9);
 
     // contract constants can be an easy alternative to enums
     const StateUnpaid: Int = 0;