docs: fix simple typo, strutures -> structures

[timgates42]

There is a small typo in st.h.

Should read structures rather than strutures.

Semi-automated pull request generated by
https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[codecov-io]

Codecov Report

Merging #154 (29082a9) into master (a06a42b) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
- Coverage   82.86%   82.79%   -0.08%     
==========================================
  Files          63       63              
  Lines       12613    12646      +33     
==========================================
+ Hits        10452    10470      +18     
- Misses       2161     2176      +15     

Impacted Files	Coverage Δ
testc.c	96.03% <0.00%> (-1.90%)	⬇️

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[coveralls]

Coverage remained the same at 80.974% when pulling 29082a9 on timgates42:bugfix_typo_structures into a06a42b on k-takata:master.

[k-takata]

Thank you.