Fix typo in directive (forgot space) which prevented the regex resolv…

…er from being documented

doc/api.rst

@@ -75,7 +75,7 @@ from user agent strings.
 
    .. warning:: Only available if |re2|_ is installed.
 
-.. class::ua_parser.regex.Resolver(Matchers)
+.. class:: ua_parser.regex.Resolver(Matchers)
 
    An advanced resolver based on |regex|_ and a bespoke implementation
    of regex prefiltering, by the sibling project `ua-rust