Fix Reference typos

collects/scribblings/reference/engine.scrbl

@@ -67,7 +67,7 @@ procedure raises an exception, then it is re-raised by
 @racket[engine-run].}
 
 
-@defproc[(engine-result [engine engine]) any]{
+@defproc[(engine-result [engine engine?]) any]{
 
 Returns the result for @racket[engine] if it has completed with a
 value (as opposed to an exception), @racket[#f] otherwise.}

collects/scribblings/reference/eval-model.scrbl

@@ -844,7 +844,7 @@ the ports, TCP connections, @|etc|, that it manages, as well as
 terminating (or suspending) its threads. A custodian that has been
 shut down cannot manage new objects.  After the current custodian is shut
 down, if a procedure is called that attempts to create a managed resource (e.g.,
-@racket[open-input-port], @racket[thread]), then the
+@racket[open-input-file], @racket[thread]), then the
 @exnraise[exn:fail:contract].
 
 A thread can have multiple managing custodians, and a suspended thread

collects/scribblings/reference/namespaces.scrbl

@@ -334,7 +334,7 @@ module was declared, unless the module declaration itself included
 assignments to the binding via @racket[set!].}
 
 
-@defproc[(namespace-syntax-introduce [stx syntax-object?]) syntax-object?]{
+@defproc[(namespace-syntax-introduce [stx syntax?]) syntax?]{
 
 Returns a syntax object like @racket[stx], except that the current
 namespace's bindings are included in the @tech{syntax object}'s