minor doc correction

Author: franckgaga

src/model/linmodel.jl

@@ -192,7 +192,7 @@ end
 Construct the model from the discrete state-space matrices `A, Bu, C, Bd, Dd` directly.
 
 See [`LinModel(::StateSpace)`](@ref) Extended Help for the meaning of the matrices. The
-arguments `Bd` and `Dd` can be the scalar `0` is there is no measured disturbances. This
+arguments `Bd` and `Dd` can be the scalar `0` if there is no measured disturbance. This
 syntax do not modify the state-space representation provided in argument (`minreal` is not
 called). Care must be taken to ensure that the model is controllable and observable. The 
 optional parameter `NT` explicitly set the number type of vectors (default to `Float64`).