update -> block N+1

Updated the slides according to the lecture where a typo was found

Author: Asamartino

content/substrate/intro/slides.md

@@ -488,7 +488,7 @@ associated with each block.
 
 ### State Transition Function
 
-$$STF = F(block_{N}, state_{N}, code_{N}): state_{N+1}$$
+$$STF = F(block_{N+1}, state_{N}, code_{N}): state_{N+1}$$
 
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