docs: Fix the EBNF for the '=' terminal symbol

The rule-body non-terminal symbol did not quote the '=' symbol.

Signed-off-by: marco <marco@styra.com>

docs/content/policy-reference.md

@@ -712,7 +712,7 @@ policy          = { rule }
 rule            = [ "default" ] rule-head { rule-body }
 rule-head       = var [ "(" rule-args ")" ] [ "[" term "]" ] [ ( ":=" | "=" ) term ]
 rule-args       = term { "," term }
-rule-body       = [ "else" [ = term ] ] "{" query "}"
+rule-body       = [ "else" [ "=" term ] ] "{" query "}"
 query           = literal { ( ";" | ( [CR] LF ) ) literal }
 literal         = ( some-decl | expr | "not" expr ) { with-modifier }
 with-modifier   = "with" term "as" term