Add a space after a colon in Gerrit-style git footer examples.

Follow up to https://crrev.com/747851 to provide good examples to
readers in the documentation.

Change-Id: I14e1aa32abce51c9ae1f7499737ff612e440733c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212962
Reviewed-by: Henrique Ferreiro <hferreiro@igalia.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771201}
diff --git a/docs/code_reviews.md b/docs/code_reviews.md
index 1fc688d1..dcb5267d 100644
--- a/docs/code_reviews.md
+++ b/docs/code_reviews.md
@@ -186,8 +186,8 @@
   * Add the reviewer's email address in the code review tool's reviewer field
     like normal.
 
-  * Add a line "Tbr:<reviewer's email>" to the bottom of the change list
-    description. e.g. `Tbr:reviewer1@chromium.org,reviewer2@chromium.org`
+  * Add a line "Tbr: <reviewer's email>" to the bottom of the change list
+    description. e.g. `Tbr: reviewer1@chromium.org,reviewer2@chromium.org`
 
   * Type a message so that the owners in the TBR list can understand who is
     responsible for reviewing what, as part of their post-commit review