Fix jvm-runtime.rst

[arimb]

add missing space in code snippet

[PeterJohnson]

A space is not required here (between -D and the option), and various examples I’ve seen online don’t include one, e.g. https://docs.gradle.org/current/userguide/build_environment.html#setting_a_system_property

[arimb]

Strange, it didn't work for me when I used it as written but did work when I added a space. Maybe it's only needed in certain terminals

[sciencewhiz]

Both versions appear to work in CMD, but only the one with a space appears to work in powershell