Update nix-local-build.rst

Fixes #8423

(cherry picked from commit 54f23799)

doc/nix-local-build.rst

@@ -125,7 +125,7 @@ versions of packages that the this snapshot specifies:
 ::
 
     packages: .
-    import https://www.stackage.org/lts-19.2/cabal.config
+    import: https://www.stackage.org/lts-19.2/cabal.config
 
 Please note that project files do not get bundled in Cabal package tarballs,
 made using e.g. ``cabal sdist``. Project files are intended for use in local