Invalid flutter command in Readme.md

[pagdot]

The Readme contains following instructions for building Leaflet:

For now the required flutter channel is master, so issue those two commands before starting building:

~$ flutter channel master
~$ flutter update

When trying to run the second one, I get following output:

$ flutter update
Could not find a command named "update".

Run 'flutter -h' (or 'flutter <command> -h') for available flutter commands and options.

Do you mean to run flutter upgrade?

[HrX03]

Oops yea, seems like i got it wrong, the correct command is upgrade indeed

[pagdot]

Ok, I'll open a PR to fix it :)