Change type annotation arrow character (#51)

* Change type annotation arrow character

Change from the arrow character used to render the return type annotation from
WIDE-HEADED RIGHTWARDS LIGHT BARB ARROW (http://www.fileformat.info/info/unicode/char/1f862/index.htm)
to the more widely supported
RIGHTWARDS ARROW (https://www.fileformat.info/info/unicode/char/2192/index.htm)

* Use ASCII arrow (->)

Following the discussion in #51

pdoc/templates/html.mako

@@ -90,7 +90,7 @@
            params = ', '.join(f.params(annotate=show_type_annotations, link=link))
            returns = show_type_annotations and f.return_annotation(link=link) or ''
            if returns:
                returns = ' 🡢\xA0' + returns
                returns = ' ->\xA0' + returns
        %>
        <span>${f.funcdef()} ${ident(f.name)}</span>(<span>${params})${returns}</span>
    </code></dt>