Add missing quote character around codegen prefix path list

The codegen-prefix path list is semicolon separated and must thus
be quoted when passed to autoconf as it otherwise breaks up the
commandline. Add missing quote character in the README documentation.

README.md

@@ -176,7 +176,7 @@ your system.
 ```
 # Configure build environment to install at /usr/local/gpdb
 # Enable CODEGEN
-./configure --enable-codegen --prefix=/usr/local/gpdb --with-codegen-prefix="/path/to/llvm;/path/to/clang
+./configure --enable-codegen --prefix=/usr/local/gpdb --with-codegen-prefix="/path/to/llvm;/path/to/clang"
 ```
 
 ## Regression tests