README.md: Fix slash in LLVM command

Change-Id: I0d69148e1b246ec90365d489b11e1954d0485576
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

README.md

@@ -244,7 +244,7 @@ http://llvm.org/docs/GettingStarted.html#git-mirror:
 
       For Linux/macOS:
 
-          cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=<installation location> -DLLVM_ENABLE_RTTI=ON ..\llvm
+          cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=<installation location> -DLLVM_ENABLE_RTTI=ON ../llvm
           make install
 
       For Windows: