Fix minor grammar in _01_background.rs (#267)

src/tutorials/_01_background.rs

@@ -100,7 +100,7 @@ In `egg`, these are represented by the [`EGraph`], [`EClass`], and
 Even small e-graphs can represent a large number of expressions, exponential in
 the number of e-nodes.
 This compactness is what makes e-graphs a compelling data structure.
-We can define what it means for _represent_ (or _contain_) a term as follows:
+We can define what it means to _represent_ (or _contain_) a term as follows:
 
 - An e-graph represents a term if any of its e-classes do.
 - An e-class represents a term if any of its e-nodes do.