Fix anchor references in UnexpectedError docs

Caught by hyperlink when I tried to make a release:

$ ./node_modules/.bin/hyperlink -ri --canonicalroot https://unexpected.js.org --skip content-type-mismatch --skip unexpected.js.org/unexpected- site-build/index.html | ./node_modules/.bin/tap-spot
Guessing --root from input files: file:///home/andreas/work/unexpected/site-build/
.......................................................................................................................,,,,,,,,,,,,,,............................................................................................................................××

  ✖ FAIL fragment-check site-build/api/UnexpectedError/index.html --> #unexpectederror-parent
  | operator: fragment-check
  | expected: id="unexpectederror-parent"
  |       at: site-build/api/UnexpectedError/index.html:162:10 <a href="#unexpectederror-parent">...</a>

  ✖ FAIL fragment-check site-build/api/UnexpectedError/index.html --> #unexpectederror-parent
  | operator: fragment-check
  | expected: id="unexpectederror-parent"
  |       at: site-build/api/UnexpectedError/index.html:162:10 <a href="#unexpectederror-parent">...</a>

  259 tests
   14 skipped
  257 passed
    2 failed

Author: papandreou

documentation/api/UnexpectedError.md

@@ -132,12 +132,12 @@ expected 'f00!' to be 'foo!'
 ## UnexpectedError.getParents()
 
 Return an array with all the ancestor errors to this error. See
-<a href="#unexpectederror-parent">parent</a> for more information.
+<a href="#unexpectederrorparent">parent</a> for more information.
 
 ## UnexpectedError.getAllErrors()
 
 Return an array with this error and all its ancestors. See
-<a href="#unexpectederror-parent">parent</a> for more information.
+<a href="#unexpectederrorparent">parent</a> for more information.
 
 ## UnexpectedError.getDiff()