Update crystal.md (#2878)

* Update crystal.md

Ref: travis-ci/travis-build#1949

* added inline comment to example

Co-authored-by: Michał Rybiński <[email protected]>

Author: bcardiff

user/languages/crystal.md

@@ -42,20 +42,21 @@ and cc [@asterite](https://github.com/asterite),
 
 ## Configuration options
 
-By default Travis CI will use the latest Crystal release. It is also possible
-to test projects against the nightly build of Crystal. To do so, set the
-`crystal` key in `.travis.yml`. For example, to test against both nightly and
-the latest release:
+By default Travis CI will use the latest Crystal stable release. It is also possible
+to test projects against the nightly or specific version build of Crystal. To do so, set the
+`crystal` key in `.travis.yml`. For example, to test against the latest stable release, the 
+most recent nightly, and a specific version stable release:
 
 ```yaml
 dist: xenial
 language: crystal
 crystal:
   - latest
   - nightly
+  - 0.35.1. # example of specific version
 ```
 {: data-file=".travis.yml"}
 
-> Note that the nightly build will only be available on Xenial and later releases 
+> Note that the nightly build will only be available on Linux environments
 
 > You can also have a look at the [Crystal](https://config.travis-ci.com/ref/language/crystal) section in our [Travis CI Build Config Reference](https://config.travis-ci.com/).