Fixup string for HED schema specification

[yarikoptic]

@VisLab correct?

Spotted while going through hits in

• DOC: make mentioning of pipeline_name consistent #2177 (comment)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.71%. Comparing base (db1c087) to head (cf3a089).
⚠️ Report is 138 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2180   +/-   ##
=======================================
  Coverage   82.71%   82.71%           
=======================================
  Files          20       20           
  Lines        1608     1608           
=======================================
  Hits         1330     1330           
  Misses        278      278           

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