build: fix small typo in cmake doxygen build

A search word spelled as "searhc" in the LLVM_DOXYGEN_SEARCHENGINE_URL cmake
variable docstring.

Patch by Daniel Otero!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243082 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed