[fix][example][multimedia]Fix readme example code

[Mutou-cat]

After testing, we found that in the original README, the line:

Load from SD card (LVGL V9, note the A: prefix)
set_image A:/sdcard/image.ezip dsc sram

does not require the prefix.

Test result
Yellow area faild！
White area success！

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud