CMakeLists.txt: tiny fix to instructions comment.

Now that our downstream picolibc changes are in the form of multiple
patches in a subdirectory, it's better to apply them with 'git am' so
they turn into separate commits, instead of 'git apply', the same way
we do it for llvm-project.

Author: statham-arm

CMakeLists.txt

@@ -47,7 +47,7 @@
 #   git -C repos clone https://github.com/llvm/llvm-project.git
 #   git -C repos/llvm-project am -k $PWD/patches/llvm-project/*.patch
 #   git -C repos clone https://github.com/picolibc/picolibc.git
-#   git -C repos/picolibc apply $PWD/patches/picolibc/*.patch
+#   git -C repos/picolibc am -k $PWD/patches/picolibc/*.patch
 #   mkdir build
 #   cd build
 #   cmake .. -GNinja -DFETCHCONTENT_SOURCE_DIR_LLVMPROJECT=../repos/llvm-project -DFETCHCONTENT_SOURCE_DIR_PICOLIBC=../repos/picolibc