Revise mode-date child document chunk labels

* Note that we use `mod-date` in the parent document and we cannot re-use it in the child.

Author: riccardoporreca

mod-date.Rmd

@@ -1,7 +1,7 @@
-```{r set-mod-date, eval=TRUE, include=FALSE}
+```{r mod-date-setenv, eval=TRUE, include=FALSE}
 Sys.setenv(KNITR_CURRENT_INPUT = knitr_current_input)
 ```
 
-```{bash mood-date, engine.opts='-l', eval=TRUE, echo=FALSE, comment="", message=FALSE}
+```{bash mod-date-print, engine.opts='-l', eval=TRUE, echo=FALSE, comment="", message=FALSE}
 printf "Last modified: %s" "$(git log -1 --format=%cd $KNITR_CURRENT_INPUT | sed 's/^[^ ]* //;s/ [^ ]*$//')"
 ```