fix: do not copy syntax is with ! (#2874)

swapped : for ! for reference syntax

Author: kunzaatko

docs/src/man/basics.md

@@ -834,8 +834,8 @@ Recapping what we have already learned,
 To get the column `:Age` from the `german` data frame you can do the following:
 
 - to copy the vector: `german[:, :Age]`, `german[:, "Age"]` or `german[:, 2]`;
-- to get a vector without copying: `german.Age`, `german."Age"`, `german[:, :Age]`,
-  `german[:, "Age"]` or `german[:, 2]`.
+- to get a vector without copying: `german.Age`, `german."Age"`, `german[!, :Age]`,
+  `german[!, "Age"]` or `german[!, 2]`.
 
 To get the first two columns as a `DataFrame`, we can index as follows:
 - to get the copied columns: `german[:, 1:2]`, `german[:, [:id, :Age]]`, or `german[:, ["id", "Age"]]`;