[vpr][placer] fixed comment's grammar

Author: Yulang Luo

vpr/src/place/net_cost_handler.cpp

@@ -80,7 +80,7 @@ static vtr::NdMatrix<float, 2> chanx_place_cost_fac({0, 0}); // [0...device_ctx.
 static vtr::NdMatrix<float, 2> chany_place_cost_fac({0, 0}); // [0...device_ctx.grid.height()-2]
 
 /**
- * @brief For each of the vector in this struct, there is one entry per cluster level net:
+ * @brief For each of the vectors in this struct, there is one entry per cluster level net:
  * [0...cluster_ctx.clb_nlist.nets().size()-1].
  * net_cost and proposed_net_cost: Cost of a net, and a temporary cost of a net used during move assessment.
  * We also use negative cost values in proposed_net_cost as a flag to indicate that