Fix a typo

Author: alexcrichton

_posts/2016-05-26-Rust-1.9.md

@@ -171,7 +171,7 @@ the `std::panic` module, described earlier, but there's a lot more too:
 
 * Raw pointers gained `as_ref()` and `as_mut()`, which returns an `Option<&T>`,
   translating null pointers into `None`.
-* `ptr::{read,write}_volitile()` allow for volitile reading and writing from a
+* `ptr::{read,write}_volitile()` allow for volatile reading and writing from a
   raw pointer.
 
 Finally, many of the types in `libcore` did not contain a `Debug`