Update erlang.jl docstring

src/univariate/continuous/erlang.jl

@@ -1,8 +1,13 @@
 """
-    Erlang(α,θ)
+    Erlang(a=1,s=1)
 
 The *Erlang distribution* is a special case of a [`Gamma`](@ref) distribution with integer shape parameter.
 
+```math
+f(x; a, s) = \\frac{x^{a-1} e^{-x/ s}}{(a-k)! s^a},
+\\quad x > 0
+```
+
 ```julia
 Erlang()       # Erlang distribution with unit shape and unit scale, i.e. Erlang(1, 1)
 Erlang(a)      # Erlang distribution with shape parameter a and unit scale, i.e. Erlang(a, 1)