minor change to adult female proportion estimation explanation

vignettes/bbtutorial.Rmd

@@ -102,7 +102,7 @@ The adult female proportion can either be fixed or estimated from counts of cows
 
 If the user provides a value to `adult_female_proportion`, it is fixed.
 The default value is 0.65, which accounts for higher mortality of males [@smith_2004].
-If `adult_female_proportion = NULL`, the adult female proportion is estimated.
+If `adult_female_proportion = NULL`, the adult female proportion is estimated from the data (i.e., `Cows ~ Binomial(adult_female_proportion, Cows + Bulls)`).
 By default, a biologically informative prior of `Beta(65,35)` is used. 
 This corresponds to an expected value of 65%.