Fix warning in regex

[cthoyt]

I get warnings like the following because there's an unescaped slash. It either needs to be \\. or make the string a raw string, which is preferred for regex legibility.

.venv/lib/python3.10/site-packages/linkml_runtime/utils/pattern.py:34
  /home/runner/work/go-fastapi/go-fastapi/.venv/lib/python3.10/site-packages/linkml_runtime/utils/pattern.py:34: DeprecationWarning: invalid escape sequence '\.'
    var_name = re.compile("{([a-z0-9_-]+([\.-_ ][a-z0-9]+)*)}", re.IGNORECASE)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d45970a) 62.09% compared to head (b6faaa9) 62.09%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #289   +/-   ##
=======================================
  Coverage   62.09%   62.09%           
=======================================
  Files          63       63           
  Lines        8461     8461           
  Branches     2170     2170           
=======================================
  Hits         5254     5254           
  Misses       2599     2599           
  Partials      608      608           

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