Made cargo new create library instead of main (#1061)

NEAR doesn't use main.rs. In the tutorial it tells us to do a normal `cargo new` and  to delete the main.rs file and add a lib.rs file. `Cargo new` has an option to make it create a library right off the bat. I think it would be less confusing for people to not have to delete and remake a new file right off the bat.

Author: nathan-barry

docs/develop/contracts/rust/intro.md

@@ -142,26 +142,24 @@ contract using `cargo`. To create the repository, navigate back to your projects
 the following commands:
 
 ```bash
-$ cargo new rust-counter-tutorial
+$ cargo new rust-counter-tutorial --lib
 $ cd rust-counter-tutorial
 ```
 
 The first command creates a new directory called `rust-counter-tutorial`. We’ve named the project
  `rust-counter-tutorial`, and Cargo creates its files in a directory of the same name.
 
 If you check the `rust-counter-tutorial` directory, you’ll see that Cargo has generated two 
-files and one directory for us: a `Cargo.toml` file and a `src` directory with a `main.rs` file 
+files and one directory for us: a `Cargo.toml` file and a `src` directory with a `lib.rs` file 
 inside. 
 
 ```bash
 .
 ├── Cargo.toml
 └── src
-   └── main.rs
+   └── lib.rs
 ```
 
-> **Note:** The `main.rs` is not needed for this example, so feel free to delete it.
-
 ## Creating the files {#creating-the-files}
 
 This smart contract project starts out with a simple layout:
@@ -215,9 +213,9 @@ overflow-checks = true
 
 </details>
 
-### Creating `lib.rs` {#creating-librs}
+### Editing `lib.rs` {#editing-librs}
 
-Create a `./src/lib.rs` file in your text editor, and paste the content of the following
+Open the `./src/lib.rs` file in your text editor, and paste the content of the following
 [`lib.rs`](https://github.com/near-examples/rust-counter/blob/master/contract/src/lib.rs) file.
 This example uses a `lib.rs` file with the smart contract logic using a `struct`,
 the `struct`'s functions, and unit tests. This will all be in one file for this simple example.