[flang][NFC] fix comment typo in SetLowerBound description

Author: jeanPerier

flang/include/flang/Runtime/descriptor.h

@@ -60,7 +60,7 @@ class Dimension {
     return *this;
   }
   // Do not use this API to cause the LB of an empty dimension
-  // to anything other than 1.  Use SetBounds() instead if you can.
+  // to be anything other than 1.  Use SetBounds() instead if you can.
   Dimension &SetLowerBound(SubscriptValue lower) {
     raw_.lower_bound = lower;
     return *this;