readme: fix build link

issue: when clicking on this link from local, it redirects to the github file.
solution: redirect to local file.
pros: it will also helps when clicking the link on gitlab to not be redirected to github.
Change-Id: I2b8cdc83837b003b067d04eb7f42c3fdedc5cb42

README.md

@@ -28,7 +28,7 @@ For detailed information on using DHTNet, consult our documentation:
 
 Get started with DHTNet by building and installing the library:
 
-- [Build and Install Instructions](https://github.com/savoirfairelinux/dhtnet/blob/master/BUILD.md)
+- [Build and Install Instructions](BUILD.md)
 
 ## Usage Example