fix: just the small bug (#216)

The page will be modified as per #131

Author: novusnota

pages/book/functions.mdx

@@ -31,7 +31,7 @@ Extension functions allow you to implement extensions for any possible type.
 > The name of the first argument MUST be named `self` and the type of this argument is the type you are extending.
 
 ```tact
-extends fun pow(self: Int, c: Int) {
+extends fun pow(self: Int, c: Int): Int {
   let res: Int = 1;
   repeat(c) {
     res *= self;