DOC: lowercase format string in unique-id

Update configuration.txt to use the correct format string in the
unique-id documentation.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index a3b7479..f566d2f 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -7676,7 +7676,7 @@
 
   Example:
 
-        unique-id-format %{+X}o\ %Ci:%Cp_%Fi:%Fp_%Ts_%rt:%pid
+        unique-id-format %{+X}o\ %ci:%cp_%fi:%fp_%Ts_%rt:%pid
 
         will generate:
 
@@ -7696,7 +7696,7 @@
 
   Example:
 
-        unique-id-format %{+X}o\ %Ci:%Cp_%Fi:%Fp_%Ts_%rt:%pid
+        unique-id-format %{+X}o\ %ci:%cp_%fi:%fp_%Ts_%rt:%pid
         unique-id-header X-Unique-ID
 
         will generate: