Fix the usage help to respect the correct usage of the --jobs option

[RobertAudi]

The --jobs options requires an = sign to work. Without the equal sign, the number of jobs is interpreted as the ruby version to install.

The problem is described in more details in #328

[simi]

:+1: it is a little confusing, this will make it clear

[havenwood]

The other option would be to change the parsing so it doesn't require the equals. I thought the short and long form flags could just be passed through to make, but as noted the long form follows the equals convention.

If we leave it as is, it's a great idea to document. I'd lean toward fixing it instead, so --jobs 8 works rather than --jobs=8.

[cbandy]

Looks like this conflicts with https://github.com/postmodern/ruby-install/commit/0a683f0e60a7804a7066b608a56f1c46c2e0cd93 which has a similar fix.

[postmodern]

Fixed in 0a683f0e60a7804a7066b608a56f1c46c2e0cd93 and 3e37a2efee060c5b5a8366118f40853a408de88d.