Fix MisusedWeekYear bug pattern flagged by error prone

Passing "YYYYMMdd-HHmmss" format to SimpleDateFormat ctor was failing
with:

error: [MisusedWeekYear] Use of "YYYY" (week year) in a date pattern
  without "ww" (week in year). You probably meant to use "yyyy" (year) instead.
          new SimpleDateFormat("YYYYMMdd-HHmmss").format(new Date()));
          ^
    (see http://errorprone.info/bugpattern/MisusedWeekYear)
  Did you mean 'new SimpleDateFormat("yyyyMMdd-HHmmss").format(new Date()));'

Change-Id: Ie51692fbacf06aed4a8920ca6dc0d75de47a1ee8

src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java b/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
index de5014a..884d1ab 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/refprotection/BackupRef.java
@@ -188,7 +188,7 @@
     if (refName.startsWith(R_HEADS) || refName.startsWith(R_TAGS)) {
       return String.format("%s-%s",
           R_BACKUPS + refName.replaceFirst(R_REFS, ""),
-          new SimpleDateFormat("YYYYMMdd-HHmmss").format(new Date()));
+          new SimpleDateFormat("yyyyMMdd-HHmmss").format(new Date()));
     }
 
     return refName;