Fix balance_of() and this_balance() inline docs (#5586)

## Description


[This](https://forum.fuel.network/t/confusion-about-assets-on-sway/4210/1)
forum post brought up the `balance_of()` inline documentation had an
error. This has been resolved in the PR.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

Author: bitzoic

sway-lib-std/src/context.sw

@@ -19,11 +19,12 @@ use ::registers::balance;
 /// # Examples
 ///
 /// ```sway
-/// use std::{context::this_balance, constants::ZERO_B256, hash::sha256, asset::mint, call_frames::contract_id};
+/// use std::{context::this_balance, constants::DEFAULT_SUB_ID, asset::mint};
 ///
 /// fn foo() {
-///     mint(ZERO_B256, 50);
-///     assert(this_balance(sha256((ZERO_B256, contract_id()))) == 50);
+///     mint(DEFAULT_SUB_ID, 50);
+///     let asset_id = AssetId::default();
+///     assert(this_balance(asset_id)) == 50);
 /// }
 /// ```
 pub fn this_balance(asset_id: AssetId) -> u64 {
@@ -44,11 +45,12 @@ pub fn this_balance(asset_id: AssetId) -> u64 {
 /// # Examples
 ///
 /// ```sway
-/// use std::{context::balance_of, constants::ZERO_B256, hash::sha256, asset::mint, call_frames::contract_id};
+/// use std::{asset::mint, call_frames::contract_id, constants::DEFAULT_SUB_ID, context::balance_of};
 ///
 /// fn foo() {
-///     mint(ZERO_B256, 50);
-///     assert(balance_of(contract_id(), sha256((ZERO_B256, contract_id()))) == 50);
+///     mint(DEFAULT_SUB_ID, 50);
+///     let asset_id = AssetId::default();
+///     assert(balance_of(contract_id(), asset_id) == 50);
 /// }
 /// ```
 pub fn balance_of(target: ContractId, asset_id: AssetId) -> u64 {