Fix a typo and a test.

Author: rxwei

include/swift/AST/DiagnosticsSema.def

@@ -2881,7 +2881,7 @@ ERROR(differentiable_attr_ambiguous_function_identifier,none,
       "ambiguous or overloaded identifier %0 cannot be used in '@differentiable' "
       "attribute", (DeclName))
 ERROR(differentiable_attr_invalid_access,none,
-      "deriavtive function %0 is required to either be public or "
+      "derivative function %0 is required to either be public or "
       "'@usableFromInline' because the original function %1 is public or "
       "'@usableFromInline'", (DeclName, DeclName))
 ERROR(differentiable_attr_result_not_differentiable,none,

test/AutoDiff/differentiable_attr_access_control.swift

@@ -20,6 +20,6 @@ private func foo3(_ x: Float) -> Float { return 1 }
 private func dfoo3(_ x: Float) -> (Float, (Float) -> Float) { return (1, {$0}) }
 
 // Error: vjp not exported.
-@differentiable(vjp: dbar1) // expected-error {{associated differentiation function 'dbar1' is required to either be public or @usableFromInline because the original function 'bar1' is public or @usableFromInline}}
+@differentiable(vjp: dbar1) // expected-error {{derivative function 'dbar1' is required to either be public or '@usableFromInline' because the original function 'bar1' is public or '@usableFromInline'}}
 public func bar1(_ x: Float) -> Float { return 1 }
 private func dbar1(_ x: Float) -> (Float, (Float) -> Float) { return (1, {$0}) }