Fix grammar in path::Path documentation

Change from "must be used with behind a pointer" to "must be used behind a pointer"

src/libstd/path.rs

@@ -1199,7 +1199,7 @@ fn into(self) -> OsString {
 /// absolute, and so on. More details about the overall approach can be found in
 /// the module documentation.
 ///
-/// This is an *unsized* type, meaning that it must always be used with behind a
+/// This is an *unsized* type, meaning that it must always be used behind a
 /// pointer like `&` or `Box`.
 ///
 /// # Examples