Update _01_background.rs (#354)

changed class to graph
fixed grammatical error

Author: awss1236

src/tutorials/_01_background.rs

@@ -193,7 +193,7 @@ These operations maintains two key (related) invariants:
 2. **Uniqueness of e-nodes**
 
    There do not exist two distinct e-nodes with the same operators and equivalent
-   children in the e-class, either in the same e-class or different e-classes.
+   children in the e-graph, either in the same e-class or different e-classes.
    This is maintained in part by the hashconsing performed by [`add`],
    and by deduplication performed by [`union`] and [`rebuild`].
 
@@ -270,7 +270,7 @@ Put together,
   equality saturation explores all possible variants of a
   program that can be derived from a set of rewrites,
   and then it extracts the best one.
-This solves the problem of choice describes above,
+This solves the problem of choice described above,
   as equality saturation essentially applies every rewrite every iteration,
   using the e-graph to avoid exponential explosion.