overview-manual/concepts.rst: fix sayhello hardcoded bindir

Replace the hardcoded /usr/bin by ${bindir}, as it should be.

Reported-by: Thomas Perrot <thomas.perrot@bootlin.com>
(From yocto-docs rev: f06601fd09d6394fcc02134f24e337a9e6170584)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 576677eae6960dbc2d2ececeba0fde5bba7bb69f)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/overview-manual/concepts.rst

@@ -2394,8 +2394,8 @@ The contents of ``sayhello_0.1.bb`` are::
    S = "${WORKDIR}/git"
 
    do_install(){
-      install -d ${D}/usr/bin
-      install -m 0700 sayhello ${D}/usr/bin
+      install -d ${D}${bindir}
+      install -m 0700 sayhello ${D}${bindir}
    }
 
 After placing the recipes in a custom layer we can run ``bitbake sayhello``