Remove broken link

Author: trey-stafford

docs/notebooks/0_introduction.ipynb

@@ -109,8 +109,7 @@
    "metadata": {},
    "source": [
     "## References\n",
-    "1. [Airborne Topographic Mapper Calibration Procedures and Accuracy Assessment](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120008479.pdf)\n",
-    "2. [Open Source Tools for Point Cloud Processing, Storage, Subsetting, and Visualization](https://sea.ucar.edu/sites/default/files/kbeam_seaconf18.pdf)\n",
+    "*  [Airborne Topographic Mapper Calibration Procedures and Accuracy Assessment](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120008479.pdf)\n",
     "\n",
     "## Related Tools\n",
     "\n",