Add backticks

Author: Planeshifter

lib/node_modules/@stdlib/assert/is-alphanumeric/README.md

@@ -48,7 +48,7 @@ var bool = isAlphaNumeric( 'abc0123456789' );
 ## Notes
 
 -   For non-string values, the function returns `false`.
--   **Alphanumeric** is defined as the characters a-zA-Z and the numeric characters 0-9.
+-   **Alphanumeric** is defined as the characters `a-zA-Z` and the numeric characters `0-9`.
 
 </section>