fix: remove sampleStart and sampleEnd comments from example in coroutine-context-and-dispatchers.md

[PetrakovichVictoria]

https://kotlinlang.org/docs/coroutine-context-and-dispatchers.html#jumping-between-threads

UPD: removed sampleStart and sampleEnd comments.

Fix this sample not being runnable:

[dkhalanskyjb]

This example isn't running correctly, but fails with

Exception in thread "main" java.security.AccessControlException: Access control exception due to security reasons in web playground: 
 access denied ("java.lang.RuntimePermission" "modifyThread")
 at java.lang.SecurityManager.checkPermission (:-1) 

[PetrakovichVictoria]

I don't know if the problem is in the coroutines here or in some restrictions in the playground. Maybe it's just worth deleting //sampleStart and sampleEnd comments and making this example non-runnable.

[dkhalanskyjb]

This example is being run as part of our tests (https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-core/jvm/test/guide/test/DispatcherGuideTest.kt#L37-L44); so, these are probably just the playground restrictions. Also, making the example non-runnable would remove the test automatically, and I don't think we want that.

[qwwdfsad]

It's https://youtrack.jetbrains.com/issue/KTL-108 unfortunately

[PetrakovichVictoria]

This example is being run as part of our tests

I also launched it before I created this PR: locally, this example passes w/o problems.

Also, making the example non-runnable would remove the test automatically, and I don't think we want that.

1. Having sampleStart and sampleEnd shows an initial will for this example to be runnable.
2. Almost all other examples from this page work (except here because these ones show some thought in general). So, having this one example non-runnable seems strange twice.
3. For me, it's not that much important will it be runnable or not, but I suppose, if we want to leave it non-runnable, then //sampleStart and //sampleEnd should be removed (and the runnability marker too). I can do this if you wish.

[dkhalanskyjb]

if we want to leave it non-runnable, then //sampleStart and //sampleEnd should be removed (and the runnability marker too).

Won't that also remove the test generated by Knit?

[PetrakovichVictoria]

I don't see any changes after running knit locally. Removed runnability and sample start/end comments.

[dkhalanskyjb]

Thanks!