eval.c: Rename has('neovim') feature to has('nvim')

To follow the convention of 'nvim' for technical references

src/nvim/eval.c

@@ -9842,7 +9842,7 @@ static void f_has(typval_T *argvars, typval_T *rettv)
     "windows",
     "winaltkeys",
     "writebackup",
-    "neovim",
+    "nvim",
     NULL
   };