Update src/testing/doc-tests.md

Co-authored-by: Dustin J. Mitchell <[email protected]>

src/testing/doc-tests.md

@@ -17,5 +17,5 @@ pub fn shorten_string(s: &str, length: usize) -> &str {
 
 * Code blocks in `///` comments are automatically seen as Rust code.
 * The code will be compiled and executed as part of `cargo test`.
-* Adding `#` in the code will hide it from the docs, but will still compile/run it.
+* Adding `# ` in the code will hide it from the docs, but will still compile/run it.
 * Test the above code on the [Rust Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3ce2ad13ea1302f6572cb15cd96becf0).