xtask: appease new clippy warning

```
warning: unnecessary hashes around raw string literal
  --> xtask/src/docs.rs:70:17
   |
70 |           indoc! {r#"
   |  _________________^
71 | |     User-Agent:*
72 | |     Disallow: /
73 | |     "#},
   | |______^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_raw_string_hashes
   = note: `#[warn(clippy::needless_raw_string_hashes)]` on by default
help: remove all the hashes around the literal
   |
70 ~         indoc! {r"
71 |     User-Agent:*
72 |     Disallow: /
73 ~     "},
   |
```

This false negative was fixed in
rust-lang/rust-clippy#11518 (the title
incorrectly says false positive).

xtask/src/docs.rs

@@ -67,10 +67,10 @@ pub fn docs(metadata: Metadata) -> Result<()> {
 
     fs::write(
         site.join("robots.txt"),
-        indoc! {r#"
+        indoc! {r"
     User-Agent:*
     Disallow: /
-    "#},
+    "},
     )
     .context("can't write robots.txt")?;