Replace mentions of system variables with correct system variables

[danielalexis]

Using the new theme generator, the system variables have a --mat before the sys.

Example (using the button component):
filled-container-color has token --sys-primary
Should be:
filled-container-color has token --mat-sys-primary

[PrajaktaB27]

Hello @danielalexis, I was taking a look at this issue and didn't find any reference to a system variable that didn't start with --mat. Is there a specific place you were looking at or is this perchance not for the code but documentation instead?

[danielalexis]

This is documentation issue (hence being posted in the material.angular.io repo), the variables on the website are --sys-** when in reality in code they are --mat-sys-**

[PrajaktaB27]

@danielalexis thank you for the clarification, I just found the buttons page with this issue! I'd like to work on this if no one else is.
Also just to confirm, the actual change is in the json files in this repo - https://github.com/angular/material2-docs-content, right?