Minor wording and heading changes.

Replace a few uses of "called" with "used" when
talking about using protocol requirements. Also
removed the subheadings in the Using Isolated
Conformances section.

Author: hborla

TSPL.docc/LanguageGuide/Concurrency.md

@@ -1670,16 +1670,14 @@ extension Person: nonisolated Equatable {
 ### Data-Race Safety for Isolated Conformances
 
 Swift prevents data races for isolated conformances
-by ensuring that protocol requirements are only called
+by ensuring that protocol requirements are only used
 on the global actor that the conformance is isolated to.
 In generic code,
 where the concrete conforming type is abstracted away,
-protocol requirements can be called through type parameters or `any` types.
+protocol requirements can be used through type parameters or `any` types.
 
 #### Using Isolated Conformances
 
-##### Generic Code
-
 A conformance requirement to `Sendable` allows generic code to send parameter
 values to concurrently-executing code.  If generic code accepts non-`Sendable`
 types, then the generic code can only use the input values from the current
@@ -1720,8 +1718,6 @@ results in an error,
 because it would allow calling the main actor isolated implementation of `perform`
 from outside the main actor.
 
-##### Dynamic Casting
-
 Generic code can check whether a value conforms to a protocol
 through dynamic casting.
 The following code has a protocol `P`,
@@ -1778,7 +1774,7 @@ so the dynamic cast fails and `perform` is not called.
 
 #### Restricting Isolated Conformances in Concurrent Code
 
-Protocol requirements can be called
+Protocol requirements can be used
 through instances of conforming types and through metatype values.
 In generic code,
 a conformance requirement to `Sendable` or `SendableMetatype`