DOC: stick-table is available in frontend sections

Fix the proxy keywords matrix to reflect that it's permitted to use
stick-table in frontend sections.

Signed-off-by: Adam Spiers <aspiers@suse.com>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 81b641e..05f0701 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1981,7 +1981,7 @@
 stick on                                  -          -         X         X
 stick store-request                       -          -         X         X
 stick store-response                      -          -         X         X
-stick-table                               -          -         X         X
+stick-table                               -          X         X         X
 tcp-check connect                         -          -         X         X
 tcp-check expect                          -          -         X         X
 tcp-check send                            -          -         X         X