deps: fix TODO in tools/icu/README.md

[srl295]

• Subsystem: deps

Fix TODO in https://github.com/nodejs/node/blob/master/tools/icu/README.md

small-icu is the default now.

[srl295]

• watch out for deps/icu-small/source/data/in/icudt*.dat :

1. that's where a 2MB binary file will be - make sure to git add it
2. don't forget to delete the old ICU's file

[Trott]

/ping @srl295 There are two TODOs in the icu README. Are either of them addressable at this time?

[srl295]

also:

it should mention pre-landing to simplify the diffs (with fix-whitespace - what I do is actually "pre-land" the patch using the Collaborator's guide onto a fresh master, then push that again otherwise the PR diff shows gratuitous whitespace diffs that aren't really part of the final land)
it should also cleanup the order of operations and mention running the license generator after 'make distclean ; configure ' so that the path to ICU is consistent