manual/adding-packages-tips.txt: reword part about package name

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-tips.txt

@@ -31,7 +31,7 @@ using the following rules:
   +BR2_PACKAGE_FOO_BAR_BOO+);
 
 * the +*.mk+ file variable prefix is the upper case _package name_
-  `.` and `-` characters substituted with `_` (e.g.:
+  with `.` and `-` characters substituted with `_` (e.g.:
   +FOO_BAR_BOO_VERSION+).