fix syntax error

[matteofrigo]

A wrong string formatting was generating an unexpected error. The problem is now fixed.

This PR solves issue #100 .

[codecov-commenter]

Codecov Report

Merging #101 into master will increase coverage by 0.06%. The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   82.79%   82.86%   +0.06%     
==========================================
  Files          67       67              
  Lines        5860     5871      +11     
  Branches      695      696       +1     
==========================================
+ Hits         4852     4865      +13     
+ Misses        819      818       -1     
+ Partials      189      188       -1     

Impacted Files	Coverage Δ
...mipy/distributions/tests/test_distribute_models.py	98.86% <90.90%> (-1.14%)	:arrow_down:
dmipy/distributions/distribute_models.py	73.04% <100.00%> (+0.69%)	:arrow_up:
dmipy/distributions/tests/test_bingham.py	91.04% <0.00%> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 706

• 12 of 12 (100.0%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.0009%) to 84.078%

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Files with Coverage Reduction	New Missed Lines	%
dmipy/distributions/tests/test_bingham.py	4	86.3%
<!--	Total:	4		-->

Totals
Change from base Build 704:	0.0009%
Covered Lines:	5053
Relevant Lines:	5871

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💛 - Coveralls

[rutgerfick]

I notice you also added a test? so it's more than just fixing a sytax error?

[matteofrigo]

I added a test to increase the coverage, otherwise it's not going to merge it