DOC: Fix minor typo in the documentation

[iglpdc]

When talking about the scaling of NIfTI images, the docs mention that scl_slope is added to the output image, but I think it should be scl_inter instead.

[coveralls]

Coverage remained the same at 96.022% when pulling 37a17c4 on iglpdc:fix-typo into 9b23a32 on nipy:master.

[codecov-io]

Codecov Report

Merging #514 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #514   +/-   ##
=======================================
  Coverage   94.05%   94.05%           
=======================================
  Files         166      166           
  Lines       22044    22044           
  Branches     2345     2345           
=======================================
  Hits        20733    20733           
  Misses        877      877           
  Partials      434      434

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[matthew-brett]

Thanks!