Update key_concepts.md (#1209) (#1237)

Link for easier navigation (it's not immediately clear where to find
more info on SimpleSequentialChain (3 clicks away)

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Co-authored-by: Larry Fisherman <l4rryfisherman@protonmail.com>

docs/modules/chains/key_concepts.md

@@ -6,6 +6,6 @@ They vary greatly in complexity and are combination of generic, highly configura
 
 ## Sequential Chain
 This is a specific type of chain where multiple other chains are run in sequence, with the outputs being added as inputs
-to the next. A subtype of this type of chain is the `SimpleSequentialChain`, where all subchains have only one input and one output,
+to the next. A subtype of this type of chain is the [`SimpleSequentialChain`](./generic/sequential_chains.html#simplesequentialchain), where all subchains have only one input and one output,
 and the output of one is therefore used as sole input to the next chain.