Fix some spelling errors (mostly in comments).

src/qe/qe_term_graph.cpp

@@ -669,7 +669,7 @@ namespace qe {
 
             // Here we could also walk equivalence classes that
             // contain interpreted values by sort and extract
-            // disequalities bewteen non-unique value
+            // disequalities between non-unique value
             // representatives.  these disequalities are implied
             // and can be mined using other means, such as theory
             // aware core minimization