Update rtl-sdr tag to include v4 support

Compile-from-source-in-Linux.md

@@ -182,7 +182,7 @@ sudo ./install_lib.sh
 <pre><code>cd /opt/build
 git clone https://github.com/osmocom/rtl-sdr.git librtlsdr
 cd librtlsdr
-git reset --hard be1d1206bfb6e6c41f7d91b20b77e20f929fa6a7
+git reset --hard 420086af84d7eaaf98ff948cd11fea2cae71734a 
 mkdir build; cd build
 cmake -Wno-dev -DDETACH_KERNEL_DRIVER=ON -DCMAKE_INSTALL_PREFIX=/opt/install/librtlsdr ..
 make -j $(nproc) install