Fix Riemann doctest

[JeffFessler]

The doctest for riemann was failing due to an x that should have been a \times.
This fixes that. And I tweaked the citations and docs while in there.
Another purpose of this is to see if the docs will build.
No changes to actual code, so no version bump.

[coveralls]

Pull Request Test Coverage Report for Build 3307533765

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.4%) to 78.972%

---

Files with Coverage Reduction	New Missed Lines	%
src/frobenius.jl	3	80.77%

Totals
Change from base Build 3264654829:	-1.4%
Covered Lines:	169
Relevant Lines:	214

---

💛 - Coveralls

[codecov]

Codecov Report

Base: 80.37% // Head: 78.97% // Decreases project coverage by -1.40% ⚠️

Coverage data is based on head (b142768) compared to base (a4e72da).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
- Coverage   80.37%   78.97%   -1.41%     
==========================================
  Files           8        8              
  Lines         214      214              
==========================================
- Hits          172      169       -3     
- Misses         42       45       +3     

Impacted Files	Coverage Δ
src/riemann.jl	100.00% <ø> (ø)
src/frobenius.jl	80.76% <0.00%> (-11.54%)	⬇️

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[JeffFessler]

Normally I would want another pair of eyes on any code changes, but since this is just minor doc tweaks I am going to merge. Hopefully the docs dev badge will work!