removing blanks from url

[mmatera]

The same as in Mathics3/mathics-django#190

This single line PR is a proposal about how to handle long urls in docstrings. With this patch

<url>:NetworkX:
     https://networkx.org/documentation/networkx-2.8.8/reference/algorithms/\
      generated/networkx.algorithms.tree.mst.minimum_spanning_edges.html
</url>

the tag is processed as

<a href=https://networkx.org/documentation/networkx-2.8.8/reference/algorithms/generated/networkx.algorithms.tree.mst.minimum_spanning_edges.html>NetworkX</a>

[rocky]

LGTM