Fix Nextion set_component_picture call (esphome#6378)

This fixes the call to the Nextion display to change the pic id from a component.
It was previously changing the attribute `val`, which is related to something else.
In addition, I've changed the parameter for picture_id to be uint_8, as Nextion requires an integer from 0 to 255 on this attribute.

esphome/components/nextion/nextion.h

@@ -82,16 +82,16 @@ class Nextion : public NextionBase, public PollingComponent, public uart::UARTDe
   /**
    * Set the picture of an image component.
    * @param component The component name.
-   * @param value The picture name.
+   * @param value The picture id.
    *
    * Example:
    * ```cpp
-   * it.set_component_picture("pic", "4");
+   * it.set_component_picture("pic", 4);
    * ```
    *
    * This will change the image of the component `pic` to the image with ID `4`.
    */
-  void set_component_picture(const char *component, const char *picture);
+  void set_component_picture(const char *component, uint8_t picture_id);
   /**
    * Set the background color of a component.
    * @param component The component name.

esphome/components/nextion/nextion_commands.cpp

@@ -197,8 +197,8 @@ void Nextion::disable_component_touch(const char *component) {
   this->add_no_result_to_queue_with_printf_("disable_component_touch", "tsw %s,0", component);
 }
 
-void Nextion::set_component_picture(const char *component, const char *picture) {
-  this->add_no_result_to_queue_with_printf_("set_component_picture", "%s.val=%s", component, picture);
+void Nextion::set_component_picture(const char *component, uint8_t picture_id) {
+  this->add_no_result_to_queue_with_printf_("set_component_picture", "%s.pic=%d", component, picture_id);
 }
 
 void Nextion::set_component_text(const char *component, const char *text) {