perlsource: fix Porting/Maintainers invocation

[mauke]

The suggested '-M' option doesn't work:

$ perl Porting/Maintainers -M ExtUtils::ParseXS
Option m is ambiguous (maintainer, module)
Porting/Maintainers: Usage:
    --maintainer M | --module M [--files]
                List modules or maintainers matching the pattern M.
                With --files, list all the files associated with them
or
    --check | --checkmani [commit | file ... | dir ... ]
                Check consistency of Maintainers.pl
                        with a file     checks if it has a maintainer
                        with a dir      checks all files have a maintainer
                        with a commit   checks files modified by that commit
                        no arg          checks for multiple maintainers
               --checkmani is like --check, but only reports on unclaimed
               files if they are in MANIFEST
or
    --opened  | file ....
                List the module ownership of modified or the listed files

Matching is case-ignoring regexp, author matching is both by
the short id and by the full name and email.  A "module" may
not be just a module, it may be a file or files or a subdirectory.
The options may be abbreviated to their unique prefixes

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• This set of changes does not require a perldelta entry.