fix call to raise-argument-error and fix docs for with-roots

pkgs/plai/gc2/private/gc-core.rkt

@@ -178,7 +178,7 @@
     (define rt (getter))
     (unless (location? rt)
       (raise-argument-error 'with-roots
-                            'location?
+                            "location?"
                             rt)))
   (parameterize ([user-specified-roots 
                   (append

pkgs/plai/scribblings/collector2.scrbl

@@ -127,7 +127,7 @@ Determines if @racket[v] is a root.
   ]}
                                                      
 @defform[(with-roots (root-var ...) expr1 expr2 ...)
-         #:contracts ([roots-expr (listof location?)])]{
+         #:contracts ([root-var location?])]{
   Evaluates each of @racket[expr1] and the @racket[expr2]s in
   in a context with additional roots, one for each of
   the @racket[root-var]s. The @racket[get-root-set] function
@@ -146,6 +146,7 @@ Determines if @racket[v] is a root.
   
   @racketblock[
     (test (with-heap (make-vector 4)
+                     (init-allocator)
                      (define f1 (gc:alloc-flat 1))
                      (define r1 (make-root 'f1 
                                            (λ () f1)