Fix incorrect command name

[izzyleung]

In the section TFENV_AUTO_INSTALL in README.md, $ terraform use <version that is not yet installed> is supposed to be $ tfenv use <version that is not yet installed>.

Fix the incorrect command name

[OJFord]

Yes, but also those two examples don't make any sense - at least not without some words?

It should be something like:

[first example as is]
will fail if the requested version is not installed, whereas:
[second example changed to:] TFENV_AUTO_INSTALL=true terraform plan
will install it automatically if necessary.

IMO.