Replace asciidoc authors convention with attribute

Replace the second line of the asciidoc reference guide that, by
convention, specifies authors with an 'author' explicit attribute.

Issue: SPR-11096

Author: Phillip Webb

src/asciidoc/index.adoc

@@ -1,5 +1,10 @@
 = Spring Framework Reference Documentation
-Rod Johnson; Juergen Hoeller; Keith Donald; Colin Sampaleanu; Rob Harrop; Thomas Risberg; Alef Arendsen; Darren Davison; Dmitriy Kopylenko; Mark Pollack; Thierry Templier; Erwin Vervaet; Portia Tung; Ben Hale; Adrian Colyer; John Lewis; Costin Leau; Mark Fisher; Sam Brannen; Ramnivas Laddad; Arjen Poutsma; Chris Beams; Tareq Abedrabbo; Andy Clement; Dave Syer; Oliver Gierke; Rossen Stoyanchev; Phillip Webb; Rob Winch
+:author: Rod Johnson; Juergen Hoeller; Keith Donald; Colin Sampaleanu; Rob Harrop; +
+         Thomas Risberg; Alef Arendsen; Darren Davison; Dmitriy Kopylenko; +
+         Mark Pollack; Thierry Templier; Erwin Vervaet; Portia Tung; Ben Hale; +
+         Adrian Colyer; John Lewis; Costin Leau; Mark Fisher; Sam Brannen; +
+         Ramnivas Laddad; Arjen Poutsma; Chris Beams; Tareq Abedrabbo; Andy Clement; +
+         Dave Syer; Oliver Gierke; Rossen Stoyanchev; Phillip Webb; Rob Winch
 
 Core support for dependency injection, transaction management, web applications, data
 access, messaging, testing and more.