Fix RST syntax

Author: p

docs/tutorials/bson-v4.txt

@@ -646,7 +646,7 @@ Ruby vs MongoDB Regular Expressions
 -----------------------------------
 
 MongoDB server uses `Perl-compatible regular expressions implemented using
-the PCRE library<http://pcre.org/>`_ and `Ruby regular expressions
+the PCRE library <http://pcre.org/>`_ and `Ruby regular expressions
 <http://ruby-doc.org/core/Regexp.html>`_ are implemented using the
 `Onigmo regular expression engine <https://github.com/k-takata/Onigmo>`_,
 which is a fork of `Oniguruma <https://github.com/kkos/oniguruma>`_.