revised a jemalloc statement in how-to-build-and-run.md

Author: kenta7777

src/how-to-build-and-run.md

@@ -42,9 +42,9 @@ debuginfo = true
 # Gives you line numbers for backtraces.
 debuginfo-lines = true
 
-# Using the system allocator (instead of jemalloc) means that tools
-# like valgrind and memcache work better.
-use-jemalloc = false
+# Link the compiler against `jemalloc`, where on Linux and OSX it should
+# override the default allocator for rustc and LLVM.
+jemalloc = false
 ```
 
 ### What is x.py?
@@ -335,4 +335,4 @@ everything up then you only need to run one command!
 
 The documentation for the rust components are found at [rustc doc].
 
-[rustc doc]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/
+[rustc doc]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/