Update catalyst_api.md

error fix

Author: TorkelE

docs/src/api/catalyst_api.md

@@ -180,7 +180,7 @@ netstoichmat
 reactionrates
 ```
 
-## (Functions to extend or modify a network)(@id api_network_extension_and_modification)
+## [Functions to extend or modify a network](@id api_network_extension_and_modification)
 `ReactionSystem`s can be programmatically extended using
 [`@add_reactions`](@ref), [`addspecies!`](@ref), [`addparam!`](@ref) and
 [`addreaction!`](@ref), or using [`ModelingToolkit.extend`](@ref) and
@@ -289,4 +289,4 @@ validate(rs::ReactionSystem, info::String="")
 ## Utility functions
 ```@docs
 symmap_to_varmap
-```
+```