Fix spelling of lark_parser dependency in etstool.py (#1831)

`lark-parser` is an invalid EDS dependency; it should be `lark_parser`.
This PR fixes that.

Prior to this fix, `python etstool.py install` failed; I've verified
that it works after the fix.

Author: mdickinson

etstool.py

@@ -92,7 +92,7 @@
     "enthought_sphinx_theme",
     "flake8",
     "flake8_ets",
-    "lark-parser",
+    "lark_parser",
     "mypy",
     "numpy",
     "Sphinx",