Fix build instructions typo

the location of install-build-pkgs was in buildtools and not tools
---------

Co-authored-by: Jesse Harwin <jesse@jharwin.com>

mkdocs/docs/setup/build.md

@@ -18,7 +18,7 @@ $ wget https://github.com/trapexit/mergerfs/releases/download/<ver>/mergerfs-<ve
 
 ```
 $ cd mergerfs
-$ sudo tools/install-build-pkgs
+$ sudo buildtools/install-build-pkgs
 $ make deb
 $ sudo dpkg -i ../mergerfs_<version>_<arch>.deb
 ```
@@ -28,7 +28,7 @@ $ sudo dpkg -i ../mergerfs_<version>_<arch>.deb
 ```
 $ su -
 # cd mergerfs
-# tools/install-build-pkgs
+# buildtools/install-build-pkgs
 # make rpm
 # rpm -i rpmbuild/RPMS/<arch>/mergerfs-<version>.<arch>.rpm
 ```