esp_partition_erase_range(): rename parameter "start_addr" to "offset"

esp_partition_erase_range(): rename parameter "start_addr" to "offset"

The name "start_addr" (which goes straight into the docs) implies
it's an absolute address while in fact it's an offset into the
partition like what's used in all the other esp_partition_*
functions.

So in order to avoid confusion make the name consistent with the
parameter names used for the other partition functions and call it
"offset".

Merges https://github.com/espressif/esp-idf/pull/3750