make a missed string into a rawstring

[sroet]

This fixes a DeprecationWarning that popped up in the running tests after merging #787

openpathsampling/engines/toy/pes.py:374
/home/sroet/github_files/openpathsampling/openpathsampling/engines/toy/pes.py:374: DeprecationWarning: invalid escape sequence \s
"""Simple double-well potential. Independent in each degree of freedom.

[codecov]

Codecov Report

Merging #1096 (ee04292) into master (2414321) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1096      +/-   ##
==========================================
- Coverage   81.76%   81.76%   -0.01%     
==========================================
  Files         142      142              
  Lines       15604    15604              
==========================================
- Hits        12759    12758       -1     
- Misses       2845     2846       +1     

Impacted Files	Coverage Δ
openpathsampling/engines/toy/pes.py	93.33% <ø> (ø)
openpathsampling/netcdfplus/cache.py	63.72% <0.00%> (-0.33%)	⬇️

---

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 2414321...ee04292. Read the comment docs.

[dwhswenson]

LGTM, thanks. We may have a few other of these that need treatment, too; see #945.