Update README.md

ubuntu20.04 make it need "clang ninja-build"

Signed-off-by: 长柳 <110147328+changliuxy@users.noreply.github.com>

Author: changliuxy

README.md

@@ -20,7 +20,7 @@ Several standard packages are needed to build the toolchain.
 
 On Ubuntu, executing the following command should suffice:
 
-    $ sudo apt-get install autoconf automake autotools-dev curl python3 libmpc-dev libmpfr-dev libgmp-dev gawk build-essential bison flex texinfo gperf libtool patchutils bc zlib1g-dev libexpat-dev
+    $ sudo apt-get install autoconf automake autotools-dev curl python3 libmpc-dev libmpfr-dev libgmp-dev gawk build-essential bison flex texinfo gperf libtool patchutils bc zlib1g-dev libexpat-dev ninja-build
 
 On Fedora/CentOS/RHEL OS, executing the following command should suffice: