doc code bug fix

Author: TorkelE

docs/src/steady_state_functionality/examples/nullcline_plotting.md

@@ -69,7 +69,7 @@ Finally, for out nullcline equations, we can use BifurcationKit's `continuation`
 using BifurcationKit
 span = (0.0, 1.2)
 function compute_nullcline(nc_sys)
-    bprob = BifurcationProblem(nc_sys, [Y => 1.0], [ps; Xval => 0.1], Xval)
+    bprob = BifurcationProblem(nc_sys, [Y => 1.0], [ps; Xpar => 0.1], Xpar)
     opts_br = ContinuationPar(p_min = span[1], p_max = span[2], dsmax = 0.01)
     return continuation(bprob, PALC(), opts_br; bothside = true)
 end
@@ -120,4 +120,4 @@ arrow_positions = [(0.25, 0.25), (0.75, 0.75), (0.35, 0.8), (0.8, 0.35), (0.02,
 foreach(pos -> plot_xy_arrow!(pos...), arrow_positions)
 plot!()
 ```
-This also works as a form of simple stability analysis, where we can see how the solution moves *away* from the unstable steady state, and *to* the stable ones.
+This also works as a form of simple stability analysis, where we can see how the solution moves *away* from the unstable steady state, and *to* the stable ones.