docs(cipher): fix typos and remove useless notions

subjects/cipher/README.md

@@ -4,9 +4,9 @@
 
 The Atbash cipher is an encryption method in which each letter of a word is replaced by its mirror letter in the alphabet.
 
-Your objective is to create a **function** named `cipher` which must return a `Result` wrapped in an `Option`. The `Result` should contain either a `boolean` or and `Error` based on the `CipherError` structure. This structure should be the error type for the **function** `cipher`.
+Your objective is to create a **function** named `cipher` which must return a `Result` wrapped in an `Option`. The `Result` should contain either a `boolean` or an `Error` based on the `CipherError` structure. This structure should be the error type for the **function** `cipher`.
 
-`cipher` should compare the original `String` with the ciphered `String`. It should return `true` if the cipher is correct. If the cipher is incorrect it should return the error type `CipherErr` with a `boolean` and the expected atbash cipher `String`.
+`cipher` should compare the original `String` with the ciphered `String`. It should return `true` if the cipher is correct. If the cipher is incorrect it should return the error type `CipherError` with a `boolean` and the expected atbash cipher `String`.
 
 ### Expected Function and structure
 
@@ -49,7 +49,3 @@ Some(Err(CipherError { validation: false, expected: "1Svool 2dliow!" }))
 None
 $
 ```
-
-### Notions
-
-- [Module std::fmt](https://doc.rust-lang.org/std/fmt/index.html)