bugfix(spi): fix readme wave image error

Fixes https://github.com/espressif/ESP8266_RTOS_SDK/issues/462

examples/peripherals/spi_master/README.md

@@ -121,6 +121,6 @@ I (1478) spi_master_example: read_data[7]: 0x2
 
     ![wave](wave_write_to_slave.png)  
 
-  - SPI_MASTER_READ_DATA_FROM
+  - SPI_MASTER_READ_DATA_FROM_SLAVE
 
     ![wave](wave_read_from_slave.png)

examples/peripherals/spi_slave/README.md

@@ -121,6 +121,6 @@ I (2741) spi_slave_example: read_data[7]: 0x77777777
 
     ![wave](wave_write_to_slave.png)  
 
-  - SPI_MASTER_READ_DATA_FROM
+  - SPI_MASTER_READ_DATA_FROM_SLAVE
 
     ![wave](wave_read_from_slave.png)

examples/peripherals/spi_slave/main/spi_slave_example_main.c

@@ -149,6 +149,7 @@ static void spi_slave_read_master_task(void *arg)
     trans.bits.addr = 32 * 1;
     trans.bits.mosi = 32 * 8;
 
+    spi_trans(HSPI_HOST, trans); // init spi slave buf
     while (1) {
         spi_slave_set_status(HSPI_HOST, &status);
         xSemaphoreTake(spi_wr_buf_done_sem, portMAX_DELAY);