Small grammar fix in rustdoc.md

Happened to be reading through

Author: pelmers

src/doc/rustdoc.md

@@ -221,7 +221,7 @@ testing this code, the `fib` function will be included (so it can compile).
 
 Running tests often requires some special configuration to filter tests, find
 libraries, or try running ignored examples. The testing framework that rustdoc
-uses is build on crate `test`, which is also used when you compile crates with
+uses is built on crate `test`, which is also used when you compile crates with
 rustc's `--test` flag. Extra arguments can be passed to rustdoc's test harness
 with the `--test-args` flag.