poky/.gitignore: updated the lines for the kernel manual

Ended up renaming these so they are all named 'kernel-manual"

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>

.gitignore

@@ -29,10 +29,9 @@ documentation/bsp-guide/bsp-guide.pdf
 documentation/bsp-guide/bsp-guide.tgz
 documentation/yocto-project-qs/yocto-project-qs.html
 documentation/yocto-project-qs/yocto-project-qs.tgz
-documentation/kernel-manual/yocto-project-kernel-manual.html
-documentation/kernel-manual/yocto-project-kernel-manual.tgz
-documentation/kernel-manual/yocto-project-kernel-manual.pdf
-documentation/kernel-manual/yocto-project-kernel-manual.tgz
+documentation/kernel-manual/kernel-manual.html
+documentation/kernel-manual/kernel-manual.tgz
+documentation/kernel-manual/kernel-manual.pdf