Update .spi.yml (awslabs#452)

Move `AWSLambdaRuntimeCore` as first package because this is where all
the doc lives. This will make `AWSLambdaRuntimeCore` the default on
Swift Package Index

Author: sebsto

.spi.yml

@@ -1,4 +1,4 @@
 version: 1
 builder:
   configs:
-    - documentation_targets: [AWSLambdaRuntime, AWSLambdaRuntimeCore]
+    - documentation_targets: [AWSLambdaRuntimeCore, AWSLambdaRuntime]