configure: Clarify help message for --enable-debug-assertions

Author: brson

configure

@@ -534,7 +534,7 @@ opt optimize-llvm 1 "build optimized LLVM"
 opt optimize-tests 1 "build tests with optimizations"
 opt libcpp 1 "build with llvm with libc++ instead of libstdc++ when using clang"
 opt llvm-assertions 1 "build LLVM with assertions"
-opt debug-assertions 0 "build with extra debug fun"
+opt debug-assertions 0 "build with debugging assertions"
 opt fast-make 0 "use .gitmodules as timestamp for submodule deps"
 opt ccache 0 "invoke gcc/clang via ccache to reuse object files between builds"
 opt local-rust 0 "use an installed rustc rather than downloading a snapshot"