Fix function exporting bug

The Perl code did not keep hash sign, nullifying effect of the strip_doc
pipe.  As a result, the function docstring would contain single space
before each non-empty line.

Markdown does not care about that (well, unless we already had 3 spaces
there in which case a block could be considered verbatim).  In POD,
however, this means verbatim plaintext.  Viewing as manpage later (by
far most common use case), the visual difference is normally too subtle
to notice, but at some point pod2html had spilled the beans.

src/bin/shellfu-doc

@@ -280,7 +280,7 @@ filter_fdoc() {
                 exit;
             }
             next unless $isdoc;
-            s/^    #//;
+            s/^    //;
             print;
         }' | strip_doc
 }