[clangd] Give an example for symbol-builder usage

`global-symbol-builder` help message mentions `-executor=<string>`
option, but doesn't give any example of what the value could be

Assuming the most popular use case to be building the whole project
index, help message should probably give an example of such usage.

Reviewers: ioeric

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D49785

llvm-svn: 338015