[Doc] Fix link to installation script

Author: laudaa

doc/build-osx.md

@@ -29,7 +29,7 @@ NOTE: Building with Qt4 is still supported, however, could result in a broken UI
 Berkeley DB
 -----------
 It is recommended to use Berkeley DB 4.8. If you have to build it yourself,
-you can use [the installation script included in contrib/](contrib/install_db4.sh)
+you can use [the installation script included in contrib/](/contrib/install_db4.sh)
 like so
 
 ```shell

doc/build-unix.md

@@ -166,7 +166,7 @@ turned off by default.  See the configure options for upnp behavior desired:
 Berkeley DB
 -----------
 It is recommended to use Berkeley DB 4.8. If you have to build it yourself,
-you can use [the installation script included in contrib/](contrib/install_db4.sh)
+you can use [the installation script included in contrib/](/contrib/install_db4.sh)
 like so
 
 ```shell