Fix docs regarding dev

[skip ci]

Author: PallHaraldsson

README.md

@@ -25,12 +25,7 @@ pkg> add Dash DashCoreComponents DashHtmlComponents DashTable
 To install the latest (stable) development version instead:
 
 ```julia
-using Pkg
-Pkg.add(PackageSpec(url="https://github.com/plotly/DashBase.jl.git"))
-Pkg.add(PackageSpec(url="https://github.com/plotly/dash-html-components.git", rev="master"))
-Pkg.add(PackageSpec(url="https://github.com/plotly/dash-core-components.git", rev="master"))
-Pkg.add(PackageSpec(url="https://github.com/plotly/dash-table.git", rev="master"))
-Pkg.add(PackageSpec(url="https://github.com/plotly/Dash.jl.git", rev="dev"))
+pkg> add Dash#dev
 ```
 
 ## Usage