user-search.txt: Do not format description of 'footer' operator as footer

'[[footer]]' intended to add an anchor with the name 'footer' to the
section where the 'footer' operator is described, but it actually marked
this paragraph as site footer, which resulted in an unwanted
(unreadable) coloring (see https://imgur.com/a/iyUKWyd). To fix this use
a different name for the anchor: footer-operator

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I91c6c6b719700d7e4763577b690de1f93511472e
diff --git a/Documentation/user-search.txt b/Documentation/user-search.txt
index cafd5ca..cc3ac42 100644
--- a/Documentation/user-search.txt
+++ b/Documentation/user-search.txt
@@ -325,7 +325,7 @@
 regular expression. The link:http://www.brics.dk/automaton/[dk.brics.automaton
 library] is used for evaluation of such patterns.
 
-[[footer]]
+[[footer-operator]]
 footer:'FOOTER'::
 +
 Matches any change that has 'FOOTER' as footer in the commit message of the