Update relative links to Linux docs

This is a followup to [1].

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1974901

TBR=sky

Change-Id: I442ffe121378607bdc5e1b16c081b8d66b138955
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980900
Reviewed-by: Thomas Anderson <[email protected]>
Commit-Queue: Thomas Anderson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#727226}
diff --git a/docs/get_the_code.md b/docs/get_the_code.md
index a25bce6..0b4e1ac 100644
--- a/docs/get_the_code.md
+++ b/docs/get_the_code.md
@@ -14,7 +14,7 @@
 * [Chrome OS](chromeos_build_instructions.md)
 * [Fuchsia](fuchsia_build_instructions.md)
 * [iOS](ios/build_instructions.md)
-* [Linux](linux_build_instructions.md)
-* [Linux Cast](linux_cast_build_instructions.md)
+* [Linux](linux/build_instructions.md)
+* [Linux Cast](linux/cast_build_instructions.md)
 * [Mac](mac_build_instructions.md)
 * [Windows](windows_build_instructions.md)