Update diagnostics_spec.rb to clarify missing %type tag comment

Author: ydah

spec/lrama/diagnostics_spec.rb

@@ -30,7 +30,7 @@
 
           %type <i> class /* comment for class */
           %nterm <str> string /* comment for string */
-          %type string_1 string_2 /* missing `%type` tag */
+          %type string_1 string_2 /* <tag> is omitted */
 
           %nonassoc tEQEQ
           %left  tPLUS tMINUS '>'