Update API.md

Co-Authored-By: Claudio Moroni <43729990+ClaudMor@users.noreply.github.com>
Co-Authored-By: Pietro Monticone <38562595+pitmonticone@users.noreply.github.com>

Author: 3 people

docs/src/API.md

@@ -2,8 +2,7 @@
 
 This page lists all exported methods, organizing them by topic (whether it is a method that acts on vertices, edges, layers, etc) and by audience: due to how it was possible to integrate multilayer graphs within the `Graphs.jl` ecosystem, some methods are intended for developers who wish to use this library just as any other `Graphs.jl` package in their code, while others are meant to be employed by the end-user.
 
-
-## End-user
+## End-User
 
 ### Nodes
 
@@ -14,7 +13,6 @@ Node
 
 ### Vertices
 
-
 ```@docs
 eltype
 MultilayerVertex
@@ -27,7 +25,6 @@ MissingVertex
 
 ### Edges
 
-
 ```@docs
 MultilayerEdge
 ME
@@ -37,7 +34,6 @@ metadata(e::AbstractMultilayerEdge)
 
 ### Subgraphs
 
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 ```@docs
 nodes(subgraph::AbstractSubGraph)
 has_vertex(layer::Layer, mv::MultilayerVertex)
@@ -139,7 +135,6 @@ get_symmetric_interlayer(
 
 ### Multilayer-specific methods
 
-
 ```@docs
 nodes(mg::MultilayerGraph)
 has_vertex(mg::AbstractMultilayerGraph, mv::MultilayerVertex)
@@ -230,7 +225,7 @@ AbstractInterlayer
 ```@docs
 ```
 
-### Multilayer-specific methods
+### Multilayer-Specific Methods
 
 ```@docs
 has_vertex(mg::M, v::T) where {T, M <: AbstractMultilayerGraph{T}}