[Docs] Add erase step to CLI write example

[drojf]

• The current example would only work on an already-erased device, and gives the impression that the tool automatically performs an erase before writing when it doesn't erase automatically (as discussed in #8)
• The --erase switch is not used because it only works on .hex files, and users may be confused if they try to use it with other file types. Using the separate erase command is more clear for this example.

[xedbg]

Thanks @drojf - its good to clarify this. Logged as DSG-4321 - will upstream this and it will pop out the next release again.

[drojf]

I forgot to add the --verify switch, which I think users would have enabled in most common use scenarios. Not sure if you want to add it in the example though.

( If you do forget to erase, and don't --verify, then pymcuprog won't give any error message making it difficult to know what went wrong )