docs: improve wording in the "Debug with gdb" section

Signed-off-by: Edwin Garcia <egarciavalle2014@gmail.com>

EN/2_mri_structure.md

@@ -112,7 +112,7 @@ When modifying the MRI source code, you can easily introduce critical problems t
 1. Write in `ruby/test.rb` what you want to check. Note that you can't use gems or extension libraries in `test.rb`.
 3. Invoke `$ make gdb` to run miniruby with gdb. If there are no problems, gdb finishes silently.
 
-`make gdb` uses `./miniruby`. If you want to debug with `./ruby`, use `make gdb-ruby` rule.
+`make gdb` uses `./miniruby`. If you want to debug with `./ruby`, use `make gdb-ruby` instead.
 
 If you want to use break points, modify the `run.gdb` file generated by the `make gdb` command.
 For example, the `b func_name` gdb command inserts a break point at the beginning of the `func_name` function.