chore: prepare for 6.0.0-b.0 release

Author: wincent

doc/command-t.txt

@@ -594,7 +594,7 @@ POSSIBILITY OF SUCH DAMAGE.
 
 HISTORY                                         *command-t-history*
 
-main (not yet released) ~
+6.0.0-b.0 (21 October 2022) ~
 
 - feat: add back |:CommandTLine| finder
   (https://github.com/wincent/command-t/pull/408).

lua/wincent/commandt/version.lua

@@ -5,6 +5,6 @@ return {
   major = 6,
   minor = 0,
   patch = 0,
-  prerelease = 'a.4',
-  version = '6.0.0-a.4',
+  prerelease = 'b.0',
+  version = '6.0.0-b.0',
 }