Fix typo

Author: navidcy

docs/src/problem.md

@@ -56,7 +56,7 @@ params = Params(α)
 
 The particular equation is so simple that it makes no difference performance-wise whether 
 we time-step it in physical or in wavenumber space. For PDEs with nonlinear terms, 
-time-stepping in wavenumbers space is much more efficient. Thus, for demonstration purposes, 
+time-stepping in wavenumber space is much more efficient. Thus, for demonstration purposes, 
 we will time-step the equation in wavenumber space, i.e.,
 
 ```math