Add alt text to Moore's Law tutorial (numpy#99)

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>
Co-authored-by: Peyton Murray <peynmurray@gmail.com>

Author: 3 people

content/mooreslaw-tutorial.md

@@ -12,9 +12,9 @@ kernelspec:
 ---
 
 # Determining Moore's Law with real data in NumPy
-![Plot of Moore's Law data](_static/01-mooreslaw-tutorial-intro.png)
+![Scatter plot of MOS transistor count per microprocessor every two years as a demonstration of Moore's Law.](_static/01-mooreslaw-tutorial-intro.png)
 
-_The number of transistors reported on a given chip plotted on a log scale in the y axis with the date of introduction on the linear scale x-axis. The blue data points are from a [transistor count table](https://en.wikipedia.org/wiki/Transistor_count#Microprocessors). The red line is an ordinary least squares prediction and the orange line is Moore's law._
+_The number of transistors reported per a given chip plotted on a log scale in the y axis with the date of introduction on the linear scale x-axis. The blue data points are from a [transistor count table](https://en.wikipedia.org/wiki/Transistor_count#Microprocessors). The red line is an ordinary least squares prediction and the orange line is Moore's law._
 
 ## What you'll do
 
@@ -346,6 +346,8 @@ plt.legend(loc="center left", bbox_to_anchor=(1, 0.5))
 plt.ylabel("# of transistors\nper microprocessor")
 ```
 
+_A scatter plot of MOS transistor count per microprocessor every two years with a red line for the ordinary least squares prediction and an orange line for Moore's law._
+
 The linear regression captures the increase in the number of transistors
 per semiconductors each year.  In 2015, semiconductor manufacturers
 claimed they could not keep up with Moore's law anymore. Your analysis