fix attribution of "Marsaglia method" for hyperspherical variates

[bob-carpenter]

Summary:

Turns out they weren't due to (Marsaglia 1972), but are just "method 2" in that paper. Change to correct citation in both the user's guide and reference manual.

Description:

Replace citations of Marsaglia 1972 with this:

Muller, M. E. "A Note on a Method for Generating Points Uniformly on N-Dimensional Spheres." Comm. Assoc. Comput. Mach. 2, 19-20, Apr. 1959.

which is the paper Marsaglia cites.

Change in:

• User's Guide
• Reference Manual

Current Version:

v2.18.0