doc: fix concurrency patterns rawhtml generation

The Makefile target was broken due to the rename of the HTML file.

R=golang-dev
CC=golang-dev
https://golang.org/cl/5918045

doc/Makefile

@@ -8,7 +8,7 @@ RAWHTML=\
 	articles/slices_usage_and_internals.rawhtml\
 	articles/laws_of_reflection.rawhtml\
 	articles/c_go_cgo.rawhtml\
-	articles/go_concurrency_patterns_timing_out_moving_on.rawhtml\
+	articles/concurrency_patterns.rawhtml\
 	articles/godoc_documenting_go_code.rawhtml\
 	articles/gobs_of_data.rawhtml\
 	articles/json_and_go.rawhtml\