docs: Fix variable name mismatch in constructor example (#505)

The code example for the Erc20 new constructor in the documentation contained a typo that would prevent it from compiling.

The function signature defined the input parameter as initial_value,but the function body attempted to use an undefined variable named initial_supply.

This change renames the parameter in the function signature from initial_value to initial_supply to match its usage throughout the function body. This makes the code example correct, consistent, and runnable.

Author: HRITHIK-SANKAR-R

versioned_docs/version-v5/basics/events.md

@@ -191,7 +191,7 @@ See this example:
 
 ```rust
 #[ink(constructor)]
-pub fn new(initial_value: Balance) -> Self {
+pub fn new(initial_supply: Balance) -> Self {
     let caller = Self::env().caller();
     let mut balances = HashMap::new();
     balances.insert(caller, initial_supply);