message: Fix broken link in README.md
diff: ccls, which originates from [cquery](https://github.com/cquery-project/cquery), is a C/C++/Objective-C language server.
  * code completion (with both signature help and snippets)
  - * [definition](src/messages/textDocument_definition.cc)/[references](src/messages/textDcument_references.cc), and other cross references
  + * [definition](src/messages/textDocument_definition.cc)/[references](src/messages/textDocument_references.cc), and other cross references
  * cross reference extensions: `$ccls/call` `$ccls/inheritance` `$ccls/member` `$ccls/vars` ...
  * formatting
  * hierarchies: [call (caller/callee) hierarchy](src/messages/ccls_call.cc), [inheritance (base/derived) hierarchy](src/messages/ccls_inheritance.cc), [member hierarchy](src/messages/ccls_member.cc)
  