Replace "gdpPercap_1972" with "1972" in "Plotting GDP increases with time" section

[Scottan]

Mistake in this section:

plotting-gdp-increases-with-time

.loc['Sweden':'Iceland','gdpPercap_1972':]
.loc['gdpPercap_1972':,['Sweden','Iceland']]
.loc[['Sweden','Iceland'],'gdpPercap_1972':]
.loc['gdpPercap_1972':,'Sweden':'Iceland']

should be:

.loc['Sweden':'Iceland','1972':]
.loc['gdpPercap_1972':,['Sweden','Iceland']]
.loc[['Sweden','Iceland'],'1972':]
.loc['gdpPercap_1972':,'Sweden':'Iceland']