Put triple ticks on their own line

In markdown triple ticks go on a line of their own. This change does not
effect the rendering in GitHub which is managing to parse this section
correctly already.

Author: tcharding

README.md

@@ -106,7 +106,9 @@ Please refer to the [`cargo` documentation](https://doc.rust-lang.org/stable/car
 We build docs with the nightly toolchain, you may wish to use the following
 shell alias to check your documentation changes build correctly.
 
-```alias build-docs='RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --features="$FEATURES" -- -D rustdoc::broken-intra-doc-links'```
+```
+alias build-docs='RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --features="$FEATURES" -- -D rustdoc::broken-intra-doc-links'
+```
 
 ## Pull Requests