docs: Add a new example for retry inside &mut self function (#118)

Signed-off-by: Xuanwo <github@xuanwo.io>

src/docs/examples/inside_mut_self.md

@@ -0,0 +1,23 @@
+Retry an async function inside `&mut self` functions.
+
+```rust
+ use anyhow::Result;
+ use backon::ExponentialBuilder;
+ use backon::Retryable;
+
+ struct Test;
+
+ impl Test {
+     async fn fetch(&self, url: &str) -> Result<String> {
+         Ok(reqwest::get(url).await?.text().await?)
+     }
+
+     async fn run(&mut self) -> Result<String> {
+         let content = (|| async { self.fetch("https://www.rust-lang.org").await })
+             .retry(ExponentialBuilder::default())
+             .when(|e| e.to_string() == "retryable")
+             .await?;
+         Ok(content)
+     }
+ }
+```

src/docs/examples/mod.rs

@@ -6,6 +6,9 @@ pub mod basic {}
 #[doc = include_str!("closure.md")]
 pub mod closure {}
 
+#[doc = include_str!("inside_mut_self.md")]
+pub mod inside_mut_self {}
+
 #[doc = include_str!("sqlx.md")]
 pub mod sqlx {}
 

src/docs/examples/with_self.md

@@ -13,6 +13,7 @@ Retry an async function which takes `&self` as receiver.
      }
  }
 
+
  #[tokio::main(flavor = "current_thread")]
  async fn main() -> Result<()> {
      let test = Test;