docs/manual: fix typo in merged-bin details

Commit 6b53649bfa ("docs/manual: add merged-bin details to customize
rootfs") added a small typo, fix that.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/customize-rootfs.adoc

@@ -31,7 +31,7 @@ When +BR2_ROOTFS_MERGED_USR+ is enabled, then the overlay must not
 Additionally, when +BR2_ROOTFS_MERGED_BIN+ is enabled, then the overlay
   must not contain the '/usr/sbin' directory, as Buildroot will create
   it as a symbolic link to '/usr/bin'. In such a situation, should the
-  overlay have any program, they should be placed in '/usr/bin'.
+  overlay have any programs, they should be placed in '/usr/bin'.
 +
 As shown in xref:customize-dir-structure[], the recommended path for
   this overlay is +board/<company>/<boardname>/rootfs-overlay+.