Fix typo in diagnostic message

rdar://66684531
(cherry picked from commit dd95577)

Author: ahatanaka

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3322,7 +3322,7 @@ def warn_attribute_dllexport_explicit_instantiation_def : Warning<
   "'dllexport' attribute ignored on explicit instantiation definition">,
   InGroup<IgnoredAttributes>;
 def warn_invalid_initializer_from_system_header : Warning<
-  "invalid constructor form class in system header, should not be explicit">,
+  "invalid constructor from class in system header, should not be explicit">,
   InGroup<DiagGroup<"invalid-initializer-from-system-header">>;
 def note_used_in_initialization_here : Note<"used in initialization here">;
 def err_attribute_dll_member_of_dll_class : Error<