fix(corelib): correct append_byte documentation example

[sewqasreedas]

The doc example for append_byte was using append_byte(0) and asserting equality with "0", which would never be true.

append_byte(0) adds a NULL byte (0x00), not the character '0' (0x30). The assert would always fail since the byte array would contain a null character, not the string "0".

Fixed by using the char literal 'A' which is more readable and idiomatic.

[reviewable-StarkWare]

This change is 

[orizi]

@orizi reviewed all commit messages.
Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion

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corelib/src/byte_array.cairo line 182 at r1 (raw file):

    /// let mut ba = "";
    /// ba.append_byte(0x41); // Appending a single byte 'A'
    /// assert!(ba == "A");

Suggestion:

    /// ba.append_byte('A'); // Appending a single byte 'A'
    /// assert!(ba == "A");

[orizi]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion

---

corelib/src/byte_array.cairo line 182 at r1 (raw file):

    /// let mut ba = "";
    /// ba.append_byte(0x41); // Appending a single byte 'A'
    /// assert!(ba == "A");

Suggestion:

    /// let mut ba = "";
    /// ba.append_byte('A');
    /// assert!(ba == "A");

[orizi]

@orizi reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @sewqasreedas)