Update long_run_growth.md

Change "GPD" to "GDP"

Author: orectique

lectures/long_run_growth.md

@@ -130,7 +130,7 @@ colors = cm.Dark2(np.linspace(0, 0.8, len(country_names)))
 color_mapping = {country: color for country, color in zip(country_names, colors)}
 ```
 
-## GPD plots
+## GDP plots
 
 Looking at the United Kingdom we can first confirm we are using the correct country code