Fix missing space in error message.

[anntzer]

Currently the error render as "... of typestr or None..."

[codecov-io]

Codecov Report

Merging #632 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #632   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          31       31           
  Lines        3177     3177           
=======================================
  Hits         3104     3104           
  Misses         73       73           

Impacted Files	Coverage Δ
sphinx_gallery/gen_gallery.py	96.09% <100.00%> (ø)

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[lucyleeow]

Thanks @anntzer!