qapi/migration: Rewrap to 70 columns

Avoid

In file included from ../publish/qapi/qapi-schema.json:53:
.../qapi/migration.json:1750:1: documentation line longer than 70 characters

Fixes: ae00f0088f ("migration/qmp: Update "resume" flag doc in "migrate" command")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

qapi/migration.json

@@ -1747,10 +1747,11 @@
 # @detach: this argument exists only for compatibility reasons and is
 #     ignored by QEMU
 #
-# @resume: when set, use the new uri/channels specified to resume paused
-#     postcopy migration.  This flag should only be used if the previous
-#     postcopy migration was interrupted.  The command will fail unless
-#     migration is in "postcopy-paused" state.  (default: false, since 3.0)
+# @resume: when set, use the new uri/channels specified to resume
+#     paused postcopy migration.  This flag should only be used if
+#     the previous postcopy migration was interrupted.  The command
+#     will fail unless migration is in "postcopy-paused" state.
+#     (default: false, since 3.0)
 #
 # Features:
 #