Fix typo in directive (forgot space) which prevented the regex resolver from being documented

Author: masklinn

doc/api.rst

@@ -75,11 +75,12 @@ 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
-   <https://github.com/ua-parser/uap-rust`_.
+   An advanced resolver based on `Rust's regex
+   <https://github.com/rust-lang/regex>`_ and a bespoke implementation
+   of regex prefiltering, by the sibling project `uap-rust
+   <https://github.com/ua-parser/uap-rust>`_.
 
    Sufficiently fast that a cache may not be necessary, and may even
    be detrimental at smaller cache sizes