incorporate feedback from Sra

In an email:

> Typos: 
> 1. Under Sample Application, "where we can where we can create"; thus, "where we can" got repeated twice
> 2. Under Views, "ERB templates are used insert Ruby code"; it's missing "to"
> 3. The Diff under  `<%= link_to category.name, category %>` does not take to a page where readers see the code-difference (like the way Diff were behaving for other such diff links). Not sure if I am explaining this properly. So, please let me know and I can try to put up some screen-shots. 
> 4. The `<div class="form-group">` should not be highlighted as it is already there in the file. It's under Forms: `<div class="form-group">`
> 5. In the Forms, after you created the coffee-table antique, you have a piece of code. But, you have not described what's the code for, why it's there or anything like you were doing for all the codes above it. It suddenly appeared there. So, perhaps you can add something before it. Not sure if 5. will be here or go under Suggestions. 
> 
> 
> Suggestions
> 1. It would be a good idea to tell the readers that they need to create a new file called category.rb under app/models. Same with categories_controllers.rb. Newbies might be very confused otherwise; they will be searching everywhere for the file. 
> 2. There were not directories "category(ies)" in app/views where we could add the show.html.erb. So, I created a directory and then added the file inside the directory. I thought the directory should have been created when I used rails g but apparently didn't. Again the newbies might get confused. So, perhaps tell the readers to create the directory and the file. 
> 
> Questions/Clarifications
> 1. Does the database now has two tables -- categories and listings? Or is categories a part of the listing? By table, I usually mean a structure with rows and columns (in layman's word, two excel files -- categories and listing. Each of the excel file has all those columns like title of type string, name of type string, etc). So, what will you do, if you don't want to create a new table, but want to add columns to an existing table? 

I think I addressed all of it, but could use another set of eyes.