doc: fix malformed label in shields doc

Doc label at the top was malformed and couldn't be referenced.

Signed-off-by: David B. Kinder <[email protected]>

Author: dbkinder

samples/shields/shields.rst

@@ -1,4 +1,4 @@
-.. shields-samples:
+.. _shields-samples:
 
 Shields Samples
 ###############