docs: fix inline code rendering for command in RDoc

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

README.rdoc

@@ -26,7 +26,7 @@ If you are using Windows, you must install the {MSYS2}[https://www.msys2.org/] d
 1. Run the following command in your command prompt:
     $ ridk install
 2. Select option `3` (MSYS2 and MINGW development toolchain).
-3. Once the compiler installation is complete, run the `ridk exec fluent-gem install fluent-plugin-mongo` command.
+3. Once the compiler installation is complete, run the <tt>ridk exec fluent-gem install fluent-plugin-mongo</tt> command.
 
 = Plugins