introduction/index.md: fix word choice for clarity (#142)

A "slippery experience" is not a common phrase, and brings to mind something challenging, hard to grasp, or likely to lose control of at any moment. A "smooth experience" is a common phrase, meaning no rough edges, encountering no snags or friction or unexpected pitfalls.

docs/introduction/index.md

@@ -87,4 +87,4 @@ main configuration is completely shared, and the configuration modified on any h
 seamlessly synchronized to other hosts through Git.
 
 Nix almost completely shielded me from the differences between OS and architecture at the
-bottom of the three machines, and the experience was very slippery!
+bottom of the three machines, and the experience was very smooth!