documentation: fix installation section in README

The previous version of installation instructions in README were
a bit incorrect resulting in inconvinient error message on trying to
build the module. Even though the error was not that hard to spot it
ruined the experience when you can just copy and paste installation
instructions from README and get everything working

Author: Oleg Koshovets

README.md

@@ -26,8 +26,9 @@ Clone this repository and then build it using CMake:
 
 ``` bash
 git clone https://github.com/tarantool/memcached.git
-cd memcached && git submodule update --init --recursive
-cd memcached && cmake . -DCMAKE_BUILD_TYPE=RelWithDebugInfo -DCMAKE_INSTALL_PREFIX=/usr
+cd memcached
+git submodule update --init --recursive
+cmake . -DCMAKE_BUILD_TYPE=RelWithDebugInfo -DCMAKE_INSTALL_PREFIX=/usr
 make
 make install
 ```