draft-v7: Fix ANTLR representation of fixed-size buffer (#587)

Author: neNerower

standard/unsafe-code.md

@@ -863,7 +863,7 @@ Fixed-size buffers are only permitted in struct declarations and may only occur
 ```ANTLR
 fixed_size_buffer_declaration
     : attributes? fixed_size_buffer_modifier* 'fixed' buffer_element_type
-      fixed_size_buffer_declarator+ ';'
+      fixed_size_buffer_declarators ';'
     ;
 
 fixed_size_buffer_modifier
@@ -879,6 +879,10 @@ buffer_element_type
     : type
     ;
 
+fixed_size_buffer_declarators
+    : fixed_size_buffer_declarator (',' fixed_size_buffer_declarator)*
+    ;
+
 fixed_size_buffer_declarator
     : identifier '[' constant_expression ']'
     ;