doc/installation: use --bindir on td-agent v3 RPM/DEB

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

doc/installation.md

@@ -61,15 +61,18 @@ $ fluent-tailcheck --help
 ### RPM/DEB (Linux)
 
 ```console
-$ sudo td-agent-gem install fluent-tail_checker
+$ sudo td-agent-gem install fluent-tail_checker --bindir /opt/td-agent/embedded/bin
 ```
 
 Then, you can use the command as follows.
 
 ```console
-$ /opt/td-agent/embedded/lib/ruby/gems/2.4.0/bin/fluent-tailcheck --help
+$ /opt/td-agent/embedded/bin/fluent-tailcheck --help
 ```
 
+Note: On td-agent v3, you need to specify `--bindir /opt/td-agent/embedded/bin` to install the executable under `/opt/td-agent/embedded/bin`.
+Without this, the executable will be installed under `/opt/td-agent/embedded/lib/ruby/gems/2.4.0/bin/`.
+
 ### .msi (Windows)
 
 `Td-agent Command Prompt` with Administrator privilege: