xbzrle: update xbzrle doc

Add new parameter description, also:
1. Remove unsociable space.
2. Nit picking: s/two/2 in report

Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Message-Id: <20200320143216.423374-1-maozhongyi@cmss.chinamobile.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
diff --git a/migration/migration.c b/migration/migration.c
index c1d88ac..4b26110 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1243,7 +1243,7 @@
         error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
                    "xbzrle_cache_size",
                    "is invalid, it should be bigger than target page size"
-                   " and a power of two");
+                   " and a power of 2");
         return false;
     }