fix spelling mistake found by CI

Author: ArnoStrouwen

docs/src/solvers/NonlinearSystemSolvers.md

@@ -64,7 +64,7 @@ features, but have a bit of overhead on very small problems.
     likely fail.
   - `FastShortcutNonlinearPolyalg()`: The default method. A polyalgorithm that mixes fast methods
     with fallbacks to robust methods to allow for solving easy problems quickly without sacrificing
-    robustnes on the hard problems.
+    robustness on the hard problems.
   - `GeneralBroyden()`: Generalization of Broyden's Quasi-Newton Method with Line Search and
     Automatic Jacobian Resetting. This is a fast method but unstable when the condition number of
     the Jacobian matrix is sufficiently large.