Minor fixes for lecture 02 (#8)

Author: nmheim

docs/Project.toml

@@ -1,4 +1,5 @@
 [deps]
+AbstractTrees = "1520ce14-60c1-5f80-bbc7-55ef81b5835c"
 BenchmarkTools = "6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf"
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 DocumenterTools = "35a29f4d-8980-5a13-9543-d66fff28ecb8"

docs/src/lecture_01/lab.md

@@ -197,7 +197,7 @@ You can check that it is a subtype of the `Function` abstract type, with the sub
 ```@repl lab01_base
 typeof(polynomial) <: Function
 ```
-These concepts will be expanded further in the type [lecture](@ref type_lecture), however for now note that this construction is quite useful for example if we wanted to create derivative rules for our function `derivativeof(::typeof(polynomial), ...)`.
+These concepts will be expanded further in the type [lecture](@ref type_system), however for now note that this construction is quite useful for example if we wanted to create derivative rules for our function `derivativeof(::typeof(polynomial), ...)`.
 
 Looking at mathematical operators `+`, `*`, we can see that in Julia they are also standalone functions. 
 ```@repl lab01_base
@@ -340,7 +340,7 @@ As opposed to the basic definition of `a` type the array is filled with `Float64
 typeof(at), eltype(at)
 polynomial(at, x)
 ```
-With round brackets over a fixed length vector we get the `Tuple` type, which is so called immutable "array" of a fixed size (its elements cannot be changed, unless initialized from scratch). Each element can be of a different type, but here we have only one and thus the `Tuple` is aliased into `NTuple`. There are some performance benefits for using immutable structure, which will be discussed [later](@ref type_lecture) or [even later](@ref perf_lecture).
+With round brackets over a fixed length vector we get the `Tuple` type, which is so called immutable "array" of a fixed size (its elements cannot be changed, unless initialized from scratch). Each element can be of a different type, but here we have only one and thus the `Tuple` is aliased into `NTuple`. There are some performance benefits for using immutable structure, which will be discussed [later](@ref type_system) or [even later](@ref perf_lecture).
 
 
 Defining `key=value` pairs inside round brackets creates a structure called `NamedTuple`, which has the same properties as `Tuple` and furthermore its elements can be conveniently accessed by dot syntax, e.g. `ant.a₀`.