Fix a few typos in the existing docs.

pkgs/racket-doc/scribblings/inside/eval.scrbl

@@ -192,6 +192,8 @@ Applies the procedure @var{f} to the given arguments.
 Beware that the procedure can mutate @var{args} if it is the same as
 the result of @cpp{scheme_current_argument_stack}.}
 
+If @var{c} is @cpp{0}, then @var{args} can be @cpp{NULL}.
+
 @function[(Scheme_Object* scheme_apply_multi
            [Scheme_Object* f]
            [int c]

pkgs/racket-doc/scribblings/inside/ports.scrbl

@@ -697,7 +697,7 @@ Creates a Scheme input file port from an ANSI C file pointer. The file
            [const-char* filename]
            [const-char* who])]{
 
-Opens @var{filename} for reading. In an exception is raised, the
+Opens @var{filename} for reading. If an exception is raised, the
  exception message uses @var{who} as the name of procedure that raised
  the exception.}