fix example isa(1m, Length) (#454)

`isa(1m, Length)` did not work after `using Unitful` but the corrected version works.

docs/src/types.md

@@ -31,7 +31,7 @@ Finally, we define physical quantity types as [`Quantity{T<:Number, D, U}`](@ref
 quantity, staged functions can be used to offload as much of the unit
 computation to compile-time as is possible. By also having the dimensions
 explicitly in the type signature, dispatch can be done on dimensions:
-`isa(1m, Length) == true`. This works because `Length` is a type alias for
+`isa(1u"m", Unitful.Length) == true`. This works because `Length` is a type alias for
 some subset of `Unitful.Quantity` subtypes.
 
 ## API