Pod::Html: fix anchorify() whitespace stripping

[iabyn]

This method had the line

$anchor =~ s/^\s+//; s/\s+$//;      # Strip white space.

where leading space was stripped from $anchor, but trailing space was being attempted to be stripped from $_ instead, giving rise to a bunch of warnings like:

Use of uninitialized value $_ in substitution (s///)
    at lib/Pod/Html/Util.pm line 253.

during 'make install.html'

TODO: fill description here

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• This set of changes requires a perldelta entry, and it is included.
• This set of changes requires a perldelta entry, and I need help writing it.
• This set of changes does not require a perldelta entry.