Further typo fix in ruleconfig.md

Co-authored-by: Seth Axen <seth.axen@gmail.com>

Author: Jutho

docs/src/rule_author/superpowers/ruleconfig.md

@@ -85,7 +85,7 @@ struct EnzymeRuleConfig <: RuleConfig{Union{SupportsMutation, HasReverseMode, No
 ```
 
 Note: you can only depend on the presence of a feature, not its absence.
-This means we may need to define features and their complements, when one is not the obvious default (as in the fast of [`HasReverseMode`](@ref)/[`NoReverseMode`](@ref) and [`HasForwardsMode`](@ref)/[`NoForwardsMode`](@ref).).
+This means we may need to define features and their complements, when one is not the obvious default (as in the case of [`HasReverseMode`](@ref)/[`NoReverseMode`](@ref) and [`HasForwardsMode`](@ref)/[`NoForwardsMode`](@ref).).
 
 
 Such special properties generally should only be defined in `ChainRulesCore`.