libext2fs: fix a minor grammatical error in the error catalog

'an block' should be 'a block'.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/po/zh_CN.po b/po/zh_CN.po
index edbcc27..77f4a22 100644
--- a/po/zh_CN.po
+++ b/po/zh_CN.po
@@ -5644,7 +5644,7 @@
 
 #: lib/ext2fs/ext2_err.c:40
 #, fuzzy
-msgid "Can't write an block bitmap"
+msgid "Can't write a block bitmap"
 msgstr "块位图"
 
 #: lib/ext2fs/ext2_err.c:41