Update broken relative links in Linux instructions

The Linux build instructions have been moved to a subdirectory, update
some of the relative paths that have been broken.

Bug: None
Change-Id: Ic22296624f12cf8d50f1b63ec037179d7930f47f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001750
Auto-Submit: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731767}

docs/linux/build_instructions.md

 # Checking out and building Chromium on Linux
 
 There are instructions for other platforms linked from the
-[get the code](get_the_code.md) page.
+[get the code](../get_the_code.md) page.
 
 ## Instructions for Google Employees
 
@@ -331,7 +331,7 @@ other settings):
 
 ### More links
 
-*   Information about [building with Clang](clang.md).
+*   Information about [building with Clang](../clang.md).
 *   You may want to [use a chroot](using_a_chroot.md) to
     isolate yourself from versioning or packaging conflicts.
 *   Cross-compiling for ARM? See [LinuxChromiumArm](chromium_arm.md).