Update docs/handbook/overview.md

Co-authored-by: Piotr Konopka <piotr.jan.konopka@cern.ch>

Author: teo

docs/handbook/overview.md

@@ -60,7 +60,7 @@ Callback execution is further refined with integer indexes, with the syntax `±i
 ## Workflow hook calls
 
 The state machine callback moments are exposed to the AliECS workflow template interface and can be used as triggers or synchronization points for integration plugin function calls. The `call` block can be used for this purpose, with similar syntax to the `task` block used for controllable tasks. Its fields are as follows.
-* `func` - mandatory, it parses as an `antonmedv/expr` expression that corresponds to a call to a function that belongs to an integration plugin object (e.g. `bookkeeping.StartOfRun()`, `dcs.EndOfRun()`, etc.).
+* `func` - mandatory, it parses as an [`antonmedv/expr`](https://github.com/antonmedv/expr) expression that corresponds to a call to a function that belongs to an integration plugin object (e.g. `bookkeeping.StartOfRun()`, `dcs.EndOfRun()`, etc.).
 * `trigger` - mandatory, the expression at `func` will be executed once the state machine reaches this moment.
 * `await` - optional, if absent it defaults to the same as `trigger`, the expression at `func` needs to finish by this moment, and the state machine will block until `func` completes.
 * `timeout` - optional, Go `time.Duration` expression, the maximum time `func` will be granted to complete before its context is invalidated.