Update manifest-reference.md

Co-authored-by: Milan Curcic <[email protected]>

Author: LKedward

manifest-reference.md

@@ -373,7 +373,7 @@ Multiple external modules can be specified as a list.
 
 ```toml
 [build]
-external-modules = ["netcdff","h5lt"]
+external-modules = ["netcdf", "h5lt"]
 ```
 
 ## Automatic target discovery