[ifp] fix broken link (#34)

Author: mmcky

lectures/ifp.md

@@ -43,8 +43,7 @@ This is an essential sub-problem for many representative macroeconomic models
 * {cite}`Huggett1993`
 * etc.
 
-It is related to the decision problem in the {doc}`stochastic optimal growth
-model <optgrowth>` and yet differs in important ways.
+It is related to the decision problem in the {doc}`stochastic optimal growth model <optgrowth>` and yet differs in important ways.
 
 For example, the choice problem for the agent includes an additive income term that leads to an occasionally binding constraint.