Drop --resolver nightly from Stack instructions

I doubt this is necessary.

Author: Ericson2314

doc/obtaining.rst

@@ -19,7 +19,7 @@ From `Hackage <https://hackage.haskell.org/package/alex>`__ via `Cabal Install <
 
 From `Stackage <https://www.stackage.org/package/alex>`__ via `Stack <https://haskellstack.org>`__::
 
-   $ stack install --resolver nightly alex
+   $ stack install alex
 
 Moreover, recent versions of ``cabal`` will automatically install the required version of ``alex`` based on ``build-tools``/``build-tool-depends`` `declarations <http://cabal.readthedocs.io/en/latest/developing-packages.html#pkg-field-build-tool-depends>`__.