[Test] Fix capitalisation of 'FileCheck'

Author: karwa

test/Serialization/nested-protocols.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-swift-frontend -emit-module -module-name nestedprotocolsource -emit-module-path %t/nestedprotocolsource.swiftmodule -primary-file %S/Inputs/nested-protocols.swift
-// RUN: %target-swift-frontend -emit-sil %s -I %t | Filecheck %s
+// RUN: %target-swift-frontend -emit-sil %s -I %t | FileCheck %s
 import nestedprotocolsource
 
 // CHECK: usesNested<A>(_:)