flower-field: Avoid confusion in hint

Using "char" as an abbreviation in this context can lead to confusion.
It could be interpreted as the Rust type "char". The size of that type
is always 4 bytes. The sentence refers to a character as it is encoded
in the UTF-8 string.

Motivated by this forum post:
https://forum.exercism.org/t/flower-field-explains-char-different-from-rust-docs/20304

Author: senekor

exercises/practice/flower-field/.docs/instructions.append.md

@@ -3,7 +3,7 @@
 ## Performance Hint
 
 All the inputs and outputs are in ASCII.
-Rust `String`s and `&str` are utf8, so while one might expect `"Hello".chars()` to be simple, it actually has to check each char to see if it's 1, 2, 3 or 4 `u8`s long.
+Rust `String`s and `&str` are utf8, so while one might expect `"Hello".chars()` to be simple, it actually has to check each character to see if it's 1, 2, 3 or 4 `u8`s long.
 If we know a `&str` is ASCII then we can call `.as_bytes()` and refer to the underlying data as a `&[u8]` (byte slice).
 Iterating over a slice of ASCII bytes is much quicker as there are no codepoints involved - every ASCII byte is one `u8` long.