README tells contributors to use a run-its profile that does not exist

[slachiewicz]

README.md line 62 tells contributors:

Run all the tests with mvn -Prun-its verify to assure nothing else was accidentally broken.

There is no run-its profile in any POM in this repository. A contributor following the instruction gets a build that quietly runs fewer tests than they think, since Maven only warns about a missing profile.

Two things would make the instruction true:

• mvn verify is what actually runs the normal suite.
• The tests that the instruction is probably reaching for are the SSH ones, which are excluded by default and need -Dssh-tests -Dssh-embedded=true. Those did not run at all until [wagon-3.x] Revive the embedded SSH tests and run them in CI #904.

Worth correcting alongside #904, since the right command depends on whether that lands.

Found while writing the developer guide in #905, which documents the working commands.