Update Javadoc for functionId

[delabassee]

Javadoc still uses the 'old' functionId pattern, they should be updated to reflect the 'new' functionId style.

E.g.

• fdk-java/flow-api/src/main/java/com/fnproject/fn/api/flow/Flow.java

  Line 34 in 0b7d2e6

  * Function IDs should be of the form "APPID/path/in/app" (without leading slash) where APPID may either be a named application or ".", indicating the appID of the current (calling) function.

• fdk-java/flow-api/src/main/java/com/fnproject/fn/api/flow/Flow.java

  Line 80 in 0b7d2e6

  * @param functionId Function ID of function to invoke - this should be the function ID returned by `fn inspect function appName fnName`

• fdk-java/flow-api/src/main/java/com/fnproject/fn/api/flow/Flow.java

  Line 201 in 0b7d2e6

  * return rt.invokeAsync("testapp/testfn","input".getBytes()).thenApply(String::new);

• fdk-java/flow-api/src/main/java/com/fnproject/fn/api/flow/Flow.java

  Line 223 in 0b7d2e6

  * return rt.invokeAsync("testapp/testfn","input".getBytes()).thenApply(String::new);

• ...