docs: Improve wording in dev install guide
docs: Improve wording in dev install guide

Refer to "2.x" and "3.x" rather than "2.1" and "3.0" since this ensures
the document will be future-proof ("2.1" is already wrong - it should
have said "2.2").

Add a reference to the upgrading doc.

Signed-off-by: James O. D. Hunt <348fbaadfbada55ea105602bf5609e23ac1c4898@intel.com>

clearcontainers/runtime,devimc/runtime,devimc/runtime,chavafg/runtime,clearcontainers/runtime,chavafg/runtime