dev-manual: Updated EXTRA_IMAGE_FEATURES operater for read-only-rootfs example

The operator used was "=" which was inconsistent in light of
previous uses of the variable in the ptest section.  I changed the
operator to "+=" to be consistent.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
(From yocto-docs rev: 2cc73ae4e3a543a60bcc29b54d2de97b08f990db)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/documentation/dev-manual/dev-manual-common-tasks.xml b/documentation/dev-manual/dev-manual-common-tasks.xml
index 65a6b6fe33..2c949908c5 100644
--- a/documentation/dev-manual/dev-manual-common-tasks.xml
+++ b/documentation/dev-manual/dev-manual-common-tasks.xml
@@ -5166,7 +5166,7 @@ the details.
                 </literallayout>
                 or
                 <literallayout class='monospaced'>
-     EXTRA_IMAGE_FEATURES = "read-only-rootfs"
+     EXTRA_IMAGE_FEATURES += "read-only-rootfs"
                 </literallayout>
             </para>