Update the tutorial for writing a contract (#90)

docs/tutorials/write-a-contract.mdx

@@ -16,7 +16,7 @@ well suited to being deployed into small programs like those deployed to
 blockchains.
 
 ```rust
-use soroban_sdk::{contractimpl, vec, Env, Symbol, Vec};
+use soroban_sdk::{contractimpl, symbol, vec, Env, Symbol, Vec};
 ```
 
 The contract will need to import the types and macros that it needs from the
@@ -26,13 +26,13 @@ how to setup a project.
 Many of the types available in typical Rust programs, such as `std::vec::Vec`,
 are not available, as there is no allocator and no heap memory in Soroban
 contracts. The `soroban-sdk` provides a variety of types like `Vec`, `Map`,
-`BigInt`, `Binary`, `FixedBinary`, that all utilize the Soroban environment's
-memory and native capabilities.
+`BigInt`, `Bytes`, `BytesN`, `Symbol`, that all utilize the Soroban
+environment's memory and native capabilities.
 
 ```rust
 pub struct Contract;
 
-#[contractimpl(export_if = "export")]
+#[contractimpl]
 impl Contract {
     pub fn hello(env: Env, to: Symbol) -> Vec<Symbol> {
         todo!()
@@ -46,22 +46,18 @@ externally are should be marked with `pub` visibility. The first argument can be
 an `Env` argument to get a copy of the Soroban environment, which is necessary
 for most things.
 
-Implementations annotated can be configured to export the contract functions
-only if a feature is enabled, with `export_if = "[feature-name]"`.
-
 Putting those pieces together a simple contract will look like this.
 
 ```rust title="src/lib.rs"
 #![no_std]
-use soroban_sdk::{contractimpl, vec, Env, Symbol, Vec};
+use soroban_sdk::{contractimpl, symbol, vec, Env, Symbol, Vec};
 
 pub struct Contract;
 
-#[contractimpl(export_if = "export")]
+#[contractimpl]
 impl Contract {
     pub fn hello(env: Env, to: Symbol) -> Vec<Symbol> {
-        const GREETING: Symbol = Symbol::from_str("Hello");
-        vec![&env, GREETING, to]
+        vec![&env, symbol!("Hello"), to]
     }
 }
 ```