update and fix bug in ipynb file

tutorials/Python/AOP/Hyperspectral/intro-hyperspectral/plot-spectral-signature/plot_spectral_signatures.ipynb

@@ -73,14 +73,12 @@
     "<figure>\n",
     "\t<a href=\"https://raw.githubusercontent.com/NEONScience/NEON-Data-Skills/main/graphics/hyperspectral-general/vegetationSpectrum_Roman2016.png\">\n",
     "\t<img src=\"https://raw.githubusercontent.com/NEONScience/NEON-Data-Skills/main/graphics/hyperspectral-general/vegetationSpectrum_Roman2016.png\"></a>\n",
-    "\t<figcaption> Spectral signature of vegetation. Source: Roman, Anamaria & Ursu, Tudor. (2016). Multispectral satellite imagery and airborne laser scanning techniques for the detection of archaeological vegetation marks. \n",
+    "\t<figcaption> Spectral signature of vegetation. Source: Mark Elowitz \n",
     "\t</figcaption>\n",
     "</figure>\n",
     "\n",
-    "Vegetation has a unique spectral signature characterized by high reflectance in the near infrared wavelengths, and much lower reflectance in the green portion of the visible spectrum. For more details, please see <a href="https://www.nv5geospatialsoftware.com/learn/whitepapers/whitepaper-detail/vegetation-analysis-using-vegetation-indices-in-envi" target="_blank">Vegetation Analysis: Using Vegetation Indices in ENVI</a> . We can extract reflectance values in the NIR and visible spectrums from hyperspectral data in order to map vegetation on the earth's surface. You can also use spectral curves as a proxy for vegetation health. We will explore this concept more in the next lesson, where we will caluclate vegetation indices. \n",
-    "\n",
-    "\n",
-    "    \n",
+    "Vegetation has a unique spectral signature characterized by high reflectance in the near infrared wavelengths, and much lower reflectance in the green portion of the visible spectrum. For more details, please see Vegetation Analysis: Using Vegetation Indices in ENVI . We can extract reflectance values in the NIR and visible spectrums from hyperspectral data in order to map vegetation on the earth's surface. You can also use spectral curves as a proxy for vegetation health. We will explore this concept more in the next lesson, where we will calculate vegetation indices.\n",
+    "  \n",
     "<figure>\n",
     "\t<a href=\"https://raw.githubusercontent.com/NEONScience/NEON-Data-Skills/main/graphics/hyperspectral-general/ReflectanceCurves_waterVegSoil.png\">\n",
     "\t<img src=\"https://raw.githubusercontent.com/NEONScience/NEON-Data-Skills/main/graphics/hyperspectral-general/ReflectanceCurves_waterVegSoil.png\"></a>\n",
@@ -89,6 +87,13 @@
     "</figure>\n"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Let's get started. First import the required packages."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 1,