Fix typo in RenderingServer docs

[Kiisu-Master]

RenderingServer.viewport_get_measured_render_time_gpu was self referencing itself in a "See also" sentence.
I believe it meant to reference the similar function for cpu instead.

[clayjohn]

Makes sense!

Usually the "see also" points to the scene-level version of the function (i.e. the Viewport method in this case). But Viewport doesn't expose this, so the intention must have been to highlight the CPU variant.

[akien-mga]

Thanks!