[flake8-async] Fix examples to use async with (#12924)

Author: MichaReiser

crates/ruff_linter/src/rules/flake8_async/rules/async_function_with_timeout.rs

@@ -32,7 +32,7 @@ use crate::rules::flake8_async::helpers::AsyncModule;
 ///
 ///
 /// async def main():
-///     with asyncio.timeout(2):
+///     async with asyncio.timeout(2):
 ///         await long_running_task()
 /// ```
 ///

crates/ruff_linter/src/rules/flake8_async/rules/cancel_scope_no_checkpoint.rs

@@ -22,14 +22,14 @@ use crate::rules::flake8_async::helpers::MethodName;
 /// ## Example
 /// ```python
 /// async def func():
-///     with asyncio.timeout(2):
+///     async with asyncio.timeout(2):
 ///         do_something()
 /// ```
 ///
 /// Use instead:
 /// ```python
 /// async def func():
-///     with asyncio.timeout(2):
+///     async with asyncio.timeout(2):
 ///         do_something()
 ///         await awaitable()
 /// ```