Reflect renaming of the plugin to 'avatars-external'

To harmonize plugin project names, the plugin's project got renamed
from 'plugins/avatars/external' (slash before 'external') to
'plugins/avatars-external' (dash before 'external').

We reflect this renaming in BUCK's build documentation.

Change-Id: Iea0bfc30533c866c2665877e7a8f1c37c8f15cdf
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/src/main/resources/Documentation/build.md b/src/main/resources/Documentation/build.md
index 801f0f9..4d8a0c3 100644
--- a/src/main/resources/Documentation/build.md
+++ b/src/main/resources/Documentation/build.md
@@ -10,7 +10,7 @@
 From gerrit source directory:
 
 ```
-buck build plugins/avatars/external:avatars-external
+buck build plugins/avatars-external
 ```
 
-You will find the `avatars-external.jar` file in `buck-out/gen/plugins/avatars/external`.
+You will find the `avatars-external.jar` file in `buck-out/gen/plugins/avatars-external`.