Fix a grammar error

Co-authored-by: Hiroki Kobayashi <[email protected]>

Author: qryxip

src/dsu.rs

@@ -4,7 +4,7 @@
 ///
 /// See: [Zvi Galil and Giuseppe F. Italiano, Data structures and algorithms for disjoint set union problems](https://core.ac.uk/download/pdf/161439519.pdf)
 ///
-/// In the following documentation, let $n$ be size of the DSU.
+/// In the following documentation, let $n$ be the size of the DSU.
 ///
 /// # Example
 ///