Update CHANGELOG.md

Author: sl-solution

CHANGELOG.md

@@ -4,7 +4,7 @@
 
 * Users now can choose between having the observations ids for the left data set and/or the right data set as part of the output data set.
 * Add a new function `eachgroup`. It allows iteration over each group of a grouped data set.
-* `op` is a new keyword argument for the `update/!` functions which allows passing a user defined function to control how the value of the main data set should be updated by the values from the transaction data set. (issue #55)
+* `op` is a new keyword argument for the `update/!` functions which allows passing a user defined function to control how the value of the main data set should be updated by the values from the transaction data set. ([issue #55](https://github.com/sl-solution/InMemoryDatasets.jl/issues/55))
 
 ## Fixes