Fix missing docstring for Map (#190)

* Update maps.jl

* Update index.md

Author: DanielVandH

docs/src/index.md

@@ -56,6 +56,9 @@ ContinuumArrays.MulPlan
 ContinuumArrays.PiecewiseBasis
 ```
 ```@docs
+ContinuumArrays.Map
+```
+```@docs
 ContinuumArrays.MappedFactorization
 ```
 ```@docs
@@ -76,4 +79,4 @@ ContinuumArrays.HvcatBasis
 ```@docs
 ContinuumArrays.ProjectionFactorization
 ```
-```
+```

src/maps.jl

@@ -10,7 +10,6 @@ is `union(m)`.
 Maps must also overload `invmap` to give the inverse of the map, which
 is equivalent to `invmap(m)[x] == findfirst(isequal(x), m)`.
 """
-
 abstract type Map{T} <: AbstractQuasiVector{T} end
 
 invmap(M::Map) = error("Overload invmap(::$(typeof(M)))")
@@ -170,4 +169,4 @@ show(io::IO, P::Union{AffineMappedQuasiVector,AffineMappedQuasiMatrix}) = print(
 function summary(io::IO, P::Union{AffineMappedQuasiVector,AffineMappedQuasiMatrix})
     summary(io, parent(P))
     print(io, " affine mapped to $(parentindices(P)[1].x.domain)")
-end
+end