Update lexical-structure.md

RexJaeschke · RexJaeschke · commit edfe37aab2ff · 2021-02-05T16:57:20.000-05:00
diff --git a/standard/lexical-structure.md b/standard/lexical-structure.md
@@ -555,7 +555,7 @@ Binary_Digit
     ;
 
 Binary_Digit_Or_Underscore
-    : binary_digit
+    : Binary_Digit
     | '_'
     ;
 ```
