Fix missing abi requirement (#3655)

#3649 added a requirement for libc++abi-dev but missed updating the
README to mention it.

README.md

@@ -291,6 +291,7 @@ sudo apt install \
   bazel \
   clang \
   libc++-dev \
+  libc++abi-dev \
   lld
 
 # Download Carbon's code.