Fixing Typo in validation error message (hyperledger#55)

Signed-off-by: Christopher Hare <chris.hare@consensys.net>

besu/src/main/java/org/hyperledger/besu/controller/BesuControllerBuilder.java

@@ -190,7 +190,7 @@ public BesuController<C> build() {
     checkNotNull(metricsSystem, "Missing metrics system");
     checkNotNull(privacyParameters, "Missing privacy parameters");
     checkNotNull(dataDirectory, "Missing data directory"); // Why do we need this?
-    checkNotNull(clock, "Mising clock");
+    checkNotNull(clock, "Missing clock");
     checkNotNull(transactionPoolConfiguration, "Missing transaction pool configuration");
     checkNotNull(nodeKeys, "Missing node keys");
     checkNotNull(storageProvider, "Must supply a storage provider");