Add alignment markers to formula in quadgk-examples.md

[abhro]

The newline renders in Documenter.jl without an align/align* environment, but it centers all the formula lines

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.36%. Comparing base (1ba958e) to head (7b4dc74).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #134   +/-   ##
=======================================
  Coverage   92.36%   92.36%           
=======================================
  Files           8        8           
  Lines         786      786           
=======================================
  Hits          726      726           
  Misses         60       60           

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