Add test image to ProfilingBlockDevice.md

Update image for testing purposes.

Author: Amanda Butler

docs/reference/api/storage/ProfilingBlockDevice.md

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 ## ProfilingBlockDevice
 
+<span class="images">![](https://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/class_profiling_block_device.png)<span>ProfilingBlockDevice class hierarchy</span></span>
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 The ProfilingBlockDevice class provides a decorator for an existing block device object to log reads, writes and erases.
 
 ProfilingBlockDevices take in a pointer to the block device being profiled as the only configurable parameter. If you want to count a storage operation such as programming, reading or writing to a block device, you should use the ProfilingBlockDevice object as the interface to the storage block rather than the underlying device. The below example highlights this use case.