Update first_simulation.md to not be overdetermined

Fixes #281

Author: ChrisRackauckas

docs/src/getting_started/first_simulation.md

@@ -52,7 +52,7 @@ number of animals at each time?
 using ModelingToolkit, DifferentialEquations, Plots
 
 # Define our state variables: state(t) = initial condition
-@variables t x(t)=1 y(t)=1 z(t)=2
+@variables t x(t)=1 y(t)=1 z(t)
 
 # Define our parameters
 @parameters α=1.5 β=1.0 γ=3.0 δ=1.0
@@ -113,12 +113,14 @@ variables:
 
 ```@example first_sim
 # Define our state variables: state(t) = initial condition
-@variables t x(t)=1 y(t)=1 z(t)=2
+@variables t x(t)=1 y(t)=1 z(t)
 ```
 
 Notice here that we use the form `state = default`, where on the right-hand side the default
-value of a state is interpreted to be its initial condition. This is then done similarly
-for parameters, where the default value is now the parameter value:
+value of a state is interpreted to be its initial condition. Note that since `z` will be given
+by an algebraic equation, we do not need to specify its initial condition.
+
+This is then done similarly for parameters, where the default value is now the parameter value:
 
 ```@example first_sim
 # Define our parameters