Update hex.md (#686)

Fix spelling typo

Author: Flamenco

src/encoding/string/hex.md

@@ -9,7 +9,7 @@ representation of the data.
 Similarly, a `HEXUPPER::decode` method is provided which takes a `&[u8]` and
 returns a `Vec<u8>` if the input data is successfully decoded.
 
-The example below coverts `&[u8]` data to hexadecimal equivalent.  Compares this
+The example below converts `&[u8]` data to hexadecimal equivalent.  Compares this
 value to the expected value.
 
 ```rust,edition2018