Fix Celsius spelling in regression lecture (#750)

Corrects the spelling of Celsius in the x-axis label.

Co-authored-by: John Stachurski <john.stachurski@gmail.com>

Author: Zhengyizhe0209-arch

lectures/simple_linear_regression.md

@@ -68,7 +68,7 @@ ax = df.plot(
     y='Y', 
     kind='scatter', 
     ylabel='Ice-cream sales ($\'s)', 
-    xlabel='Degrees celcius'
+    xlabel='Degrees Celsius'
 )
 ```