Corrected command syntax for opm migrate.

Author: subhtk

modules/opm-cli-ref-migrate.adoc

@@ -10,7 +10,7 @@ Migrate a SQLite database format index image or database file to a file-based ca
 .Command syntax
 [source,terminal]
 ----
-$ opm  <index_ref> <output_dir> [<flags>]
+$ opm migrate <index_ref> <output_dir> [<flags>]
 ----
 
 .`migrate` flags