refactor: switch default to guess in Integrator

Author: SebastianM-C

src/Blocks/continuous.jl

@@ -20,7 +20,7 @@ Initial value of integrator state ``x`` can be set with `x`
 @mtkmodel Integrator begin
     @extend u, y = siso = SISO()
     @variables begin
-        x(t) = 0.0, [description = "State of Integrator"]
+        x(t), [guess = 0.0, description = "State of Integrator"]
     end
     @parameters begin
         k = 1, [description = "Gain"]
@@ -37,8 +37,8 @@ end
 Outputs an approximate derivative of the input. The transfer function of this block is
 
 ```
-k      k        ks  
-─ - ─────── = ────── 
+k      k        ks
+─ - ─────── = ──────
 T   sT² + T   sT + 1
 ```