clarification in MCF reference (#239)

clarified format_version value needs to be a string

docs/content/reference/mcf.md

@@ -422,7 +422,7 @@ name|Mandatory|name of the online resource|Download portal|ISO 19115:2003 Sectio
 description|Mandatory|detailed text description of what the online resources is/does|brief description of the online resource (English)|ISO 19115:2003 Section B.3.2.5
 function|Mandatory|code for function performed by the online resource (must be one of 'download', 'information', 'offlineAccess', 'order', 'search')|download|ISO 19115:2003 Section B.3.2.5
 format|Optional|Format of the distribution method|WMS|HNAP 2.3
-format_version|Optional|Format version of the distribution method|1.0|HNAP 2.3
+format_version|Optional|Format version of the distribution method. Note: this value needs to be encoded as a string|1.0|HNAP 2.3
 channel|Optional|channel/topic/exchange when link is a Pub/Sub endpoint|my/cool/topic|OGC API - Pub/Sub