refactor(endpoints): rename usage_report parameter to usage_page

Aligns zmk_endpoints_send_report function declaration with its definition.

PR: #522

app/include/zmk/endpoints.h

@@ -18,4 +18,4 @@ int zmk_endpoints_select(enum zmk_endpoint endpoint);
 int zmk_endpoints_toggle();
 enum zmk_endpoint zmk_endpoints_selected();
 
-int zmk_endpoints_send_report(uint8_t usage_report);
+int zmk_endpoints_send_report(uint8_t usage_page);