replace t by s

Author: ocots

docs/src/manual-initial-guess.md

@@ -249,7 +249,7 @@ You can define Julia aliases within the `@init` block using `=` (single equals,
 ig = @init ocp2 begin
     amplitude = 0.5
     phase = sin(amplitude)
-    φ = 2π * t
+    φ = 2π * s
     q(s) := amplitude * sin(φ)
     v(s) := amplitude * cos(φ)
     u(s) := phase      # constant function using alias