change descriptive text to match param names

Author: kmid5280

src/chime_dash/app/templates/en/intro.md

@@ -1,5 +1,5 @@
 The estimated number of currently infected individuals is **{total_infections:.0f}**. This is based on current inputs for
-    Hospitalizations (**{current_hosp}**), Hospitalization rate (**{hosp_rate:.0%}**), Region size (**{S}**),
+    Hospitalizations (**{current_hosp}**), Hospitalization rate (**{hosp_rate:.0%}**), Regional population (**{S}**),
     and Hospital market share (**{market_share:.0%}**).
 
 {infected_population_warning_str}