docs: added missing ./ (#428)

Author: realbadidas

INSTALL.md

@@ -22,7 +22,7 @@ git clone https://github.com/Microsoft/vcpkg.git
 cd vcpkg
 ./bootstrap-vcpkg.sh
 ./vcpkg integrate install
-vcpkg install socket-io-client
+./vcpkg install socket-io-client
 ```
 
 The Socket.IO client port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please [create an issue or pull request](https://github.com/Microsoft/vcpkg) on the vcpkg repository.