Improve comment in closure migration code.

Author: m-ou-se

compiler/rustc_typeck/src/check/upvar.rs

@@ -648,9 +648,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let mut closure_body_span = self.tcx.hir().span(body_id.hir_id);
 
                     // If the body was entirely expanded from a macro
-                    // invocation, as the body is not contained inside the
-                    // closure span. In that case, we walk up the expansion
-                    // until we find the span before the expansion.
+                    // invocation, i.e. the body is not contained inside the
+                    // closure span, then we walk up the expansion until we
+                    // find the span before the expansion.
                     while !closure_body_span.is_dummy() && !closure_span.contains(closure_body_span) {
                         closure_body_span = closure_body_span.parent().unwrap_or(DUMMY_SP);
                     }