contributor-guide: use "apt" instead of "aptitude"

As used in the rest of the manual.

(From yocto-docs rev: 5a5dfc446bb62568da50056660d18c5c309d1051)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/contributor-guide/submit-changes.rst

@@ -57,7 +57,7 @@ Set up Git
 The first thing to do is to install Git packages. Here is an example
 on Debian and Ubuntu::
 
-   sudo aptitude install git-core git-email
+   sudo apt install git-core git-email
 
 Then, you need to set a name and e-mail address that Git will
 use to identify your commits::