Resolve "`plot.GRiwrm`: mermaid code is displayed with the diagram"

Closes #73 (closed)

vignettes/V01_Structure_SD_model.Rmd

 ---
 title: 'Severn_01: Set up of a semi-distributed GR model network'
 author: "David Dorchies"
+output: rmarkdown::html_vignette
 vignette: >
   %\VignetteIndexEntry{Severn_01: Set up of a semi-distributed GR model network}
   %\VignetteEngine{knitr::rmarkdown}

vignettes/V02_Calibration_SD_model.Rmd

 ---
 title: "Severn_02: Calibration of a GR4J semi-distributed model network"
 author: "David Dorchies"
+output: rmarkdown::html_vignette
 vignette: >
   %\VignetteIndexEntry{Severn_02: Calibration of a GR4J semi-distributed model network}
   %\VignetteEngine{knitr::rmarkdown}

vignettes/V03_Open-loop_influenced_flow.Rmd

 ---
 title: "Severn_03: Calibration of an open-loop influenced flow model network"
 author: "David Dorchies"
+output: rmarkdown::html_vignette
 vignette: >
   %\VignetteIndexEntry{Severn_03: Calibration of an open-loop influenced flow model network}
   %\VignetteEngine{knitr::rmarkdown}