fix static_functions_not_mutating error msg

Author: felixinkinto

include/swift/AST/DiagnosticsSema.def

@@ -1718,7 +1718,7 @@ ERROR(functions_mutating_and_not,none,
       "method must not be declared both %0 and %1",
       (SelfAccessKind, SelfAccessKind))
 ERROR(static_functions_not_mutating,none,
-      "static functions must not be declared mutating", ())
+      "nonmutating or mutating modifiers cannot be used on static functions because they do not operate on instance properties or self", ())
 
 ERROR(readwriter_mutatingness_differs_from_reader_or_writer_mutatingness,none,
       "%0 cannot be %1 when "