Correct heading level for "The Input Mutation Extension" (#3406)

* Update mutations.md

Correct heading level for "The Input Mutation Extension"

* Update mutations.md

Correct casing for heading

docs/general/mutations.md

@@ -88,7 +88,7 @@ Mutations with void-result go against
 
 </Note>
 
-### The Input Mutation Extension
+## The input mutation extension
 
 It is usually useful to use a pattern of defining a mutation that receives a
 single [input type](../types/input-types) argument called `input`.