Bug 1735058 - Use absolute path to mozilla-unified after possible restart of shell. r=mhentges,glandium

bootstrap.py may ask the restart of shell to reflect the change to login script.
Given the step 2 lets user to move to c:/mozilla-source instead of home,
the document should explicitly specify the absolute path in step 3.

Differential Revision: https://phabricator.services.mozilla.com/D128024

docs/setup/windows_build.rst

@@ -131,7 +131,7 @@ Now that your system is bootstrapped, you should be able to build!
 
 .. code-block:: shell
 
-    cd mozilla-unified
+    cd c:/mozilla-source/mozilla-unified
     ./mach build
     ./mach run