Fix incorrect documentation of -p command line option.

The documentation for command line arguments to MzScheme says:

-p <file> <u> <path> : requires (planet "<file>" "<user>" "<pkg>")
http://docs.plt-scheme.org/reference/running-sa.html#(part._mz-cmdline)

Running mzscheme --help says:

 -p <package> : Like -e '(require (planet "<package>")'

Experiments confirm the executable is correct and the docs are wrong.

svn: r15597

collects/scribblings/reference/startup.scrbl

@@ -136,9 +136,8 @@ flags:
        @nonterm{path} : @scheme[require]s @scheme[(lib
        @#,nontermstr{path})].}
 
-  @item{@FlagFirst{p} @nonterm{file} @nonterm{u} @nonterm{path} :
-       @scheme[require]s @scheme[(planet @#,nontermstr{file}
-       @#,nontermstr{user} @#,nontermstr{pkg})].
+  @item{@FlagFirst{p} @nonterm{package} :
+       @scheme[require]s @scheme[(planet @#,nontermstr{package})].
 
        @margin-note{Despite its name, @DFlag{script} is not usually
        used for Unix scripts. See @guidesecref["scripts"] for more