[NFC] Maintainers.rst: align email address formatting

Follow the formatting for other email addresses in this file.

Author: svenvh

clang/Maintainers.rst

@@ -321,7 +321,7 @@ OpenMP conformance
 OpenCL conformance
 ~~~~~~~~~~~~~~~~~~
 | Sven van Haastregt
-| [email protected] (email), svenvh (GitHub)
+| sven.vanhaastregt\@arm.com (email), svenvh (GitHub)
 
 
 OpenACC