Fix typo in error message (#138)

Author: mprat

src/extrapolation/extrap_prep.jl

@@ -61,7 +61,7 @@ function extrap_prep{T,N}(::Type{T}, n::Val{N})
     return Expr(:block, exprs...)
 end
 function extrap_prep{N,T<:Tuple}(::Type{T}, n::Val{N})
-    length(T.parameters) == N || return :(throw(ArgumentError("The $N-dimensional extrap configuration $T is not supported - must be a tuple of length $N (was length $(lenght(T.parameters)))")))
+    length(T.parameters) == N || return :(throw(ArgumentError("The $N-dimensional extrap configuration $T is not supported - must be a tuple of length $N (was length $(length(T.parameters)))")))
     exprs = Expr[]
     for d in 1:N
         Tdim = T.parameters[d]
@@ -84,4 +84,4 @@ function extrap_prep{S,T,N,d}(::Type{Tuple{S,T}}, n::Val{N}, dim::Val{d})
         $(extrap_prep(S, n, dim, Val{:lo}()))
         $(extrap_prep(T, n, dim, Val{:hi}()))
     end
-end
+end