Fix markdown links (#629)

* Fix markdown links

Signed-off-by: mueller-ma <mueller-ma@users.noreply.github.com>

CONTRIBUTING.md

@@ -9,7 +9,7 @@ Issue | Where to report
 Problems and feature requests for openHAB 1 addons | [openHAB1-addons](https://github.com/openhab/openhab1-addons/issues)
 Problems and feature requests for openHAB 2 addons | [openHAB2-addons](https://github.com/openhab/openhab2-addons/issues)
 Issues related to the runtime environment, IDE and packaging | [openHAB-distro](https://github.com/openhab/openhab-distro/issues)
-Issues related to the core openHAB bundles that are not from Eclipse SmartHome | [openHAB-core](https://github.com/kaikreuzer/openhab-core/issues)
+Issues related to the core openHAB bundles that are not from Eclipse SmartHome | [openHAB-core](https://github.com/openhab/openhab-core/issues)
 Issues related to Eclipse SmartHome addons and core runtime | [Eclipse SmartHome](https://github.com/eclipse/smarthome/issues)
 
 But do not worry - if you are not clear, which category your issue belongs to - we will redirect you, if necessary.
@@ -41,8 +41,7 @@ else is working on the same thing.
 
 ### Create issues...
 
-Any significant improvement should be documented as [a GitHub
-issue](https://github.com/openhab/openhab-distro/issues?labels=enhancement&page=1&state=open) before anybody
+Any significant improvement should be documented as [a GitHub issue](https://github.com/openhab/openhab-distro/issues?labels=enhancement&page=1&state=open) before anybody
 starts working on it.
 
 ### ...but check for existing issues first!
@@ -176,7 +175,7 @@ There are several exceptions to the signing requirement. Currently these are:
 
 * Step 1: learn the component inside out
 * Step 2: make yourself useful by contributing code, bugfixes, support etc.
-* Step 3: volunteer on [in the community] (https://community.openhab.org/)
+* Step 3: volunteer on [in the community](https://community.openhab.org/)
 
 Don't forget: being a maintainer is a time investment. Make sure you will have time to make yourself available.
 You don't have to be a maintainer to make a difference on the project!