[hotfix][runtime, docs] Fixed typo in SingleInputGate

Author: leesf

flink-runtime/src/main/java/org/apache/flink/runtime/io/network/partition/consumer/SingleInputGate.java

@@ -486,7 +486,7 @@ public void requestPartitions() throws IOException, InterruptedException {
 
 				// Sanity checks
 				if (numberOfInputChannels != inputChannels.size()) {
-					throw new IllegalStateException("Bug in input gate setup logic: mismatch between" +
+					throw new IllegalStateException("Bug in input gate setup logic: mismatch between " +
 							"number of total input channels and the currently set number of input " +
 							"channels.");
 				}