Added usage example for --checksum option.

doc/aria2c.1.asciidoc

@@ -3607,6 +3607,13 @@ You can specify step counter:
 aria2c -Z -P "http://host/image[A-Z:2].png"
 -------------------------------------------
 
+Verify checksum
+^^^^^^^^^^^^^^^
+
+----------------------------------------------------------------------------------------
+aria2c --checksum=sha-1=0192ba11326fe2298c8cb4de616f4d4140213837 http://example.org/file
+----------------------------------------------------------------------------------------
+
 Parallel downloads of arbitrary number of URI,metalink,torrent
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 --------------------------------------------------------------