Add EBNF reference

Author: Azganoth

docs/extended_bnf.md

@@ -1,6 +1,6 @@
 # The complete syntax of Lua
 
-Here is the complete syntax of Lua in extended BNF. As usual in extended BNF, {A} means 0 or more As, and [A] means an optional A.
+Here is the complete syntax of Lua in [EBNF](https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form).
 
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