Mention Arturo in README (#1256)

Arturo support was added in 113cd0e
(#1232). Let's mention that it's supported.

Author: EvanHahn

README.md

@@ -36,7 +36,7 @@ translators for Pygments lexers and styles.
 
 | Prefix | Language
 | :----: | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-|   A    | ABAP, ABNF, ActionScript, ActionScript 3, Ada, Agda, AL, Alloy, AMPL, Angular2, ANTLR, ApacheConf, APL, AppleScript, ArangoDB AQL, Arduino, ArmAsm, ATL, AutoHotkey, AutoIt, Awk
+|   A    | ABAP, ABNF, ActionScript, ActionScript 3, Ada, Agda, AL, Alloy, AMPL, Angular2, ANTLR, ApacheConf, APL, AppleScript, ArangoDB AQL, Arduino, ArmAsm, Arturo, ATL, AutoHotkey, AutoIt, Awk
 |   B    | Ballerina, Bash, Bash Session, Batchfile, Beef, BibTeX, Bicep, BlitzBasic, BNF, BQN, Brainfuck
 |   C    | C, C#, C++, C3, Caddyfile, Caddyfile Directives, Cap'n Proto, Cassandra CQL, Ceylon, CFEngine3, cfstatement, ChaiScript, Chapel, Cheetah, Clojure, CMake, COBOL, CoffeeScript, Common Lisp, Coq, Core, Crystal, CSS, CSV, CUE, Cython
 |   D    | D, Dart, Dax, Desktop file, Devicetree, Diff, Django/Jinja, dns, Docker, DTD, Dylan