clippy: put backticks around symlink_root in docs

```
src/export.rs:27:1: 53:2 warning: you should put `symlink_root` between ticks in the documentation, #[warn(doc_markdown)] on by default
src/export.rs:27 pub fn unexport(pin_config: &PinConfig,
src/export.rs:28                 symlink_root: Option<&str>)
src/export.rs:29                 -> Result<(), sysfs_gpio::Error> {
src/export.rs:30     if let Some(symroot) = symlink_root {
src/export.rs:31         // create symlink for each name
src/export.rs:32         for name in &pin_config.names {
                 ...
src/export.rs:27:1: 53:2 help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#doc_markdown
```

Signed-off-by: Paul Osborne <[email protected]>

Author: posborne

src/export.rs

@@ -19,7 +19,7 @@ use sysfs_gpio;
 /// actions:
 ///
 /// 1. For each GPIO name/alias, the corresponding symlink is remvoed from
-///    `/var/run/gpio/<name>` (or an alternate configured symlink_root).
+///    `/var/run/gpio/<name>` (or an alternate configured `symlink_root`).
 /// 2. The GPIO pin istself is unexported (vai /sys/class/gpio/unexport)
 ///
 /// If the GPIO was already unexported, this function will continue