[OCaml] Rename link_modules' to link_modules

Commit 434e9561 renamed link_modules to link_modules' for unclear reasons.
Based on the commit's diff, the author possibly intended to have two
functions, link_modules to bind to LLVMLinkModules and link_modules' to
bind to LLVMLinkModules2. However, there is only one function. link_modules'
appears in LLVM 3.8 onwards.

Differential Revision: https://reviews.llvm.org/D153090