grammar: Update Instantiation Grammar Tests

A change to the grammar for parsing instantiations was not
accompanied by a change to the tests.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

tree-sitter-p4/test/corpus/instantiations.txt

@@ -10,7 +10,8 @@ bool() main;
     (bool)
    )
   )
+  (arguments)
   (identifier)
- ) 
+ )
 )