rust-core: fix typo in comment

Differential Revision: https://phab.mercurial-scm.org/D7895

rust/hg-core/src/revlog.rs

@@ -14,7 +14,7 b' pub type Revision = i32;'
 /// Marker expressing the absence of a parent
 ///
 /// Independently of the actual representation, `NULL_REVISION` is guaranteed
-/// to be smaller that all existing revisions.
+/// to be smaller than all existing revisions.
 pub const NULL_REVISION: Revision = -1;
 
 /// Same as `mercurial.node.wdirrev`