doc: Fix typo in build-unix.md

Author: hebasto

doc/build-unix.md

@@ -34,7 +34,7 @@ For the default build type `RelWithDebInfo`, the default compile flags are
 
 Finally, clang (often less resource hungry) can be used instead of gcc, which is used by default:
 
-    cmake -B build -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_CC_COMPILER=clang
+    cmake -B build -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_COMPILER=clang
 
 ## Linux Distribution Specific Instructions