Makefile: fix make help

Add missing ' to fix `make help'.

This fixes the following error:

  /bin/bash: -c: line 1: unexpected EOF while looking for matching `''
  make: *** [Makefile:1180: help] Error 2

Fixes: ca6c896bdd75cf334ac9a02a6dd786415221f55e
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Steve Hay <me@stevenhay.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

Makefile

@@ -1177,7 +1177,7 @@ help:
 	@echo '  <pkg>-graph-rdepends   - Generate a graph of <pkg>'\''s reverse dependencies'
 	@echo '  <pkg>-graph-both-depends'
 	@echo '                         - Generate a graph of both <pkg>'\''s forward and'
-	@echo '                           reverse dependencies.
+	@echo '                           reverse dependencies.'
 	@echo '  <pkg>-dirclean         - Remove <pkg> build directory'
 	@echo '  <pkg>-reconfigure      - Restart the build from the configure step'
 	@echo '  <pkg>-rebuild          - Restart the build from the build step'