vim: improve the modifier highlighting

This makes the modifier highlighting stand out against the definition
keyword.

Author: compnerd

utils/vim/syntax/swift.vim

@@ -205,7 +205,7 @@ hi def link swiftTypeAliasName Identifier
 hi def link swiftTypeName Function
 hi def link swiftConstraint Special
 hi def link swiftFuncDefinition Define
-hi def link swiftDefinitionModifier Define
+hi def link swiftDefinitionModifier Operator
 hi def link swiftInOutKeyword Define
 hi def link swiftFuncKeyword Function
 hi def link swiftFuncKeywordGeneral Function