x/website: Download Go for Mac button displays "undefined (undefined MB)" on Safari

[juhokarppinen]

Minor issue on Go download page

"Download Go for Mac" button displays "undefined (undefined MB)" on Safari 14.0.3 / macOS 10.15.7:

Steps to reproduce on Safari:

• Go to golang.org/dl
• Click on button "Apple macOS" under heading "Featured downloads"
• Click on any tab under heading "2 Go install."
• The aforementioned button should now display "undefined (undefined MB)"

I tested the same flow on Chrome, and there's no issue.

[jamalc]

Cleaning up old issues. This is resolved.