rust: cleanup some white space in a dock

They seems to have sneaked there somehow.

Differential Revision: https://phab.mercurial-scm.org/D9201

rust/hg-core/src/dirstate/dirstate_tree/tree.rs

@@ -37,7 +37,7 b' use std::path::PathBuf;'
 /// Only the first two items in this list are implemented as of this commit.
 ///
 /// [1]: https://www.mercurial-scm.org/wiki/DirsCachePlan
-///       
+///
 ///
 /// # Structure
 ///