DOC: fix math rendering for some dki functions

[matthew-brett]

See: #1507

[arokem]

Jinx!

[arokem]

See #1508

[codecov-io]

Codecov Report

Merging #1509 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1509      +/-   ##
==========================================
+ Coverage   87.55%   87.55%   +<.01%     
==========================================
  Files         246      246              
  Lines       31366    31366              
  Branches     3433     3433              
==========================================
+ Hits        27463    27464       +1     
  Misses       3085     3085              
+ Partials      818      817       -1

Impacted Files	Coverage Δ
dipy/reconst/dki.py	96.82% <ø> (ø)	⬆️
dipy/core/graph.py	75% <0%> (+1.19%)	⬆️

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