doc: Fix link to doc in README.md

Fix how link to official Documentation is shown in README.md so
that rendering in github looks better.

Signed-off-by: Tommi Rantanen <tommi.rantanen@nordicsemi.no>

Author: trantanen

README.md

@@ -3,4 +3,4 @@
 The Serial Modem contains the Serial Modem application for nRF91-based devices, and is built on [nRF Connect SDK](https://www.nordicsemi.com/Products/Development-software/nRF-Connect-SDK) (NCS).
 It includes host examples in the form of the Serial Modem AT Client library and the Serial Modem AT Client Shell sample.
 
-You can find the documentation in the [Serial Modem] (https://docs.nordicsemi.com/bundle/addon-serial_modem-latest/page/index.html) documentation.
+You can find the documentation in the [Serial Modem](https://docs.nordicsemi.com/bundle/addon-serial_modem-latest/page/index.html) documentation.