loader: clean up comment and variable

Make one variable's name plural to clarify that it
contains more than one layer.  Also, clean up a
comment that wrapped prematurely to the next line.
diff --git a/loader/loader.c b/loader/loader.c
index 714f3ab..80c1ce3 100644
--- a/loader/loader.c
+++ b/loader/loader.c
@@ -4024,7 +4024,7 @@
 VkResult loader_validate_instance_extensions(
     const struct loader_instance *inst,
     const struct loader_extension_list *icd_exts,
-    const struct loader_layer_list *instance_layer,
+    const struct loader_layer_list *instance_layers,
     const VkInstanceCreateInfo *pCreateInfo) {
 
     VkExtensionProperties *extension_prop;
@@ -4052,7 +4052,7 @@
         /* Not in global list, search layer extension lists */
         for (uint32_t j = 0; j < pCreateInfo->enabledLayerCount; j++) {
             layer_prop = loader_get_layer_property(
-                pCreateInfo->ppEnabledLayerNames[j], instance_layer);
+                pCreateInfo->ppEnabledLayerNames[j], instance_layers);
             if (!layer_prop) {
                 /* Should NOT get here, loader_validate_layers
                  * should have already filtered this case out.