[MRG] use Template.safe_substitute instead of str.replace

[jeromedockes]

as was requested somewhere in the discussion of #1766 . this is purely cosmetic

#1766 (comment)

[bthirion]

LGTM --though I don't know this part of the codebase well.

[codecov]

Codecov Report

Merging #1901 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1901      +/-   ##
==========================================
+ Coverage   95.23%   95.33%   +0.09%     
==========================================
  Files         135      135              
  Lines       17321    17880     +559     
==========================================
+ Hits        16496    17046     +550     
- Misses        825      834       +9

Impacted Files	Coverage Δ
nilearn/plotting/html_surface.py	100% <100%> (ø)	⬆️
nilearn/plotting/js_plotting_utils.py	98.63% <100%> (+0.02%)	⬆️
nilearn/plotting/html_stat_map.py	98.64% <100%> (-0.01%)	⬇️
nilearn/plotting/html_connectome.py	100% <100%> (ø)	⬆️
nilearn/tests/test_init.py	94.87% <0%> (-5.13%)	⬇️
nilearn/plotting/tests/test_img_plotting.py	100% <0%> (ø)	⬆️
nilearn/plotting/tests/test_displays.py	100% <0%> (ø)	⬆️
...earn/input_data/tests/test_nifti_spheres_masker.py	100% <0%> (ø)	⬆️
nilearn/input_data/nifti_spheres_masker.py	96.55% <0%> (+0.86%)	⬆️
nilearn/plotting/displays.py	95.88% <0%> (+0.86%)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b841c90...93bed9c. Read the comment docs.

[kchawla-pi]

For future perusers of this PR/codebase, the 4sentences that explain the benefit & pitfalls are worth reading. https://docs.python.org/2/library/string.html#string.Template.safe_substitute

[kchawla-pi]

@jeromedockes I will merge this tomorrow; doesn't seem anything more needs to be done here.