Fix Readme typo for storage crate (#221)

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There is a typo in **storage** crate **README** file. instead of
`stuct`, it should be `struct`
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- [ ] Bugfix
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## What is the current behavior?
Typo mistake in README content
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Issue Number: N/A

## What is the new behavior?
Fixed typo mistake
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## Does this introduce a breaking change?

- [ ] Yes
- [ x] No

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src/storage/README.md

@@ -6,7 +6,7 @@ An ordered sequence of values that can be used in Starknet storage:
 
 ```rust
 #[storage]
-stuct Storage {
+struct Storage {
     amounts: List<u128>
 }
 ```